Surficial Geologic Map

The surficial geology of the study area was mapped through analysis of (1) borehole lithologic logs and field observation from Kappel and others (1996); (2) borehole and test-pit lithologic logs, field observations, and electromagnetic and seismic data collected during this study; (3) light detection and ranging (lidar) imagery; (4) data from the digital Soil Survey Geographic Database (U.S. Department of Agriculture, 2019); and (5) previous surficial mapping by Pair and others (2007a, b).

Shear-Wave Velocity Lines

The multichannel analysis of surface waves seismic method was applied to determine the shear-wave velocity of the shallow sediments (Park and others, 1999). Shear-wave velocity is a measure of the rigidity or stiffness of a sediment and is related to the grain-size distribution, density, and geotechnical properties (such as liquefaction potential) of that sediment (Hussien and Karray, 2015). For example, soft, liquefiable clayey silt has low shear-wave velocities, and stiff, sandy gravel has high shear-wave velocities. Shear-wave velocity helped distinguish between the effects of lithologic texture and salinity on subsurface electrical conductivity. Shear-wave velocity data were collected on November 16–18, 2021, at 18 sites along the proposed stream-relocation paths and at 3 sites in the offline settling basin area (fig. 4).

The shear-wave velocity data were collected using a single channel seismograph and an array of as many as 14 Atom vertical-component geophones. At each data-collection site, the geophones were spaced in a line 2 meters (m) apart for an initial survey length of 26 m. At the end of each line, strike plates were placed at an offset of 8 m from the first and last geophone. Anthropogenic sources of noise were minimized by waiting for vehicle traffic to pass before starting data collection and standing still during data collection. Starting at the near end of the array, a sledgehammer was used to strike the plate at least three times. Each sledgehammer impact (shot) was separated by at least 5 seconds. Shots were stacked in postprocessing. Shots were repeated at the far end of the line. A roll-along geometry was used to extend the survey by relocating the first two geophones and attached seismographs to the far end of the line with the same 2-m spacing. With this geometry, two new sets of shots were taken by maintaining the 8-m offset from the new positions of the first and last geophones. The roll-along process was repeated a total of eight times for all seismic lines for a total profile length of 58 m.

Raw data from the Atom units were processed by using the Seisimager/SW software suite version 1.27.0 from Geometrics, Inc. The common midpoint cross correlation method (Hayashi and Suzuki, 2004) was used to group correlated traces that have a common midpoint from each shot gather within the Pickwin software module version 7.1.0.0 of Seisimager/SW. Each common midpoint cross correlation gather was combined to generate a phase-velocity diagram where surface-wave phase velocities were plotted against their corresponding frequencies. For each wave frequency, there are multiple possible velocities. The slowest velocity, called the fundamental mode, was identified for each phase-velocity diagram by using automated picking in the Pickwin module. The picks were then manually corrected, cleaned, and exported to the WaveEQ module version 7.1.0.0 of Seisimager/SW as a fundamental mode dispersion curve.

Data were inverted by using the least squares method with five to eight iterations, guided by a horizontal regularization constant of 0.3. Higher regularization values cause more horizontal smoothing in the final model, and the value of 0.3 was chosen to balance smoothing with horizontal variations in shear-wave velocity. Final model plots represent the modeled shear-wave velocity structure across the length of each line. Data were recorded at a 4-m horizontal resolution and varying vertical resolution, from about 0.5 m in the shallow parts to a 5-m vertical resolution in the deep parts of the model. The shear-wave velocity data and models are available from the associated USGS data release (Terry and others, 2026).

Frequency-Domain Electromagnetic Surveys

Frequency-domain electromagnetic surveys provide a nearly continuous measurement of electrical conductivity in the shallow subsurface. The major factors affecting electrical conductivity in sediments are their silt and clay content, their saturated porosity, and the salinity of the groundwater. Frequency-domain electromagnetic surveys were conducted by using GEM–2 and DUALEM–421 surface-geophysical instruments. The GEM–2 is a multiband electromagnetic instrument consisting of a Global Positioning System (GPS) receiver, electronics, and a battery mounted to a rigid ski about 6 ft in length in which a transmitter and receiver coil are embedded at a fixed offset (5.45 ft). The transmitter emits an electromagnetic field at multiple frequencies that induces eddy currents in subsurface conductors, such as groundwater with elevated salinity, which produces secondary electromagnetic fields that are measured by the receiver. The instrument was carried flat (horizontal coplanar) at waist height (about 3.3 ft) at a comfortable walking speed (about 3 miles per hour). Data were logged in real time every 0.1 second and uploaded via Bluetooth connection to a Trimble Nomad by using Geophex WinGEM2 software. Raw binary data were exported to comma separated value format by using the Geophex EMExport software. About 57 linear mi of GEM–2 data were collected in the study area during May 24, 2018, September 27–30, 2021, and November 15–17, 2021 (fig. 5). The GEM–2 data and inversions are available from the associated USGS data release (Terry and others, 2026).

The DUALEM–421 is a multioffset electromagnetic instrument consisting of electronics and batteries mounted in a tubular mast with a total length of about 15 ft. In this study, the instrument was carried by two people to maintain a constant ground offset and orientation. This system consists of three receivers offset from the transmitter by 4 m, 2 m, and 1 m, each with two receiver coil orientations (in plane with the transmitter coil and perpendicular to the transmitter coil) that operate at a fixed frequency of 9 kilohertz. Data and GPS information were logged at a comfortable walking speed (about 3 miles per hour) with a Bluetooth-connected Android mobile device (Android LG Phoenix 5) running the DUALEM application. Raw data were exported to comma separated value format by using the application. About 19 linear mi of DUALEM–421 data were collected in the study area during November 15–18, 2021 (fig. 6). The DUALEM–421 data and inversions are available from the associated USGS data release (Terry and others, 2026).

Raw electromagnetic geophysical data are sensitive to subsurface bulk electrical conductivity, and the different frequencies and transmitter-receiver separations and orientations effectively sense different volumes of the subsurface; however, data points are not uniquely associated with a specific depth. To obtain depth-dependent electrical conductivity images, the process of geophysical inversion (iterative, numerical model-fitting approach) is required. The electromagnetic data were inverted in Aarhus Workbench version 6.5.1.0 by using appropriate import configuration and processing settings for each instrument. Terry and others (2026) provide further details of the import, processing, and inversion settings used and output as georeferenced models of subsurface earth conductivity to a depth of 49 ft bls. Due to the inherent physical limitations of the instruments, the electrical conductivity contrasts within the subsurface itself, and necessary constraints applied in the inversion, these conductivity models are blurry (smooth) and only interpretable to some fraction of the total 49 ft bls. Workbench performs a sensitivity analysis during the inversion to provide an estimate of this maximum interpretable depth at each location, known as the depth of investigation. In general, the depth of investigation was about 15 to 25 ft for the DUALEM–421 instrument, and about 10 to 20 ft for the GEM–2 instrument. Inversion results from the DUALEM–421 instrument are presented in this report as specific depth-slice maps and vertical sections of electrical conductivity. Owing to the shallow depth of investigation of the GEM–2 survey, only the cumulative instrument response (measured sum of quadrature values) is presented in this report. The GEM–2 survey provided an independent check of the shallow DUALEM–421 response and an increased areal coverage for surficial geologic mapping purposes.

Well Drilling, Sediment Sampling, and Standard Penetration Testing

Well drilling, sediment sampling, and standard penetration tests (SPTs) were completed at eight sites during April 11–14, 2022, to help characterize subsurface lithologic texture and stiffness of the sediments in the study area (fig. 4). Seven of the sites were along or near the proposed stream-relocation paths, and the eighth site was at the northwestern edge of the proposed offline settling basin area. The wells were drilled with 8-inch (in) diameter hollow-stem augers. Sediment sampling and SPTs were completed in accordance with standard procedures and equipment described by ASTM International (2022): a 2-in diameter, 1.5-ft long split-spoon sampler and 140-pound automatic hammer are used to obtain nearly continuous sediment samples and measure the resistance of the sediments to the penetration of the sampler. During each test, the number of hammer blows were recorded for three 0.5 ft intervals. The standard penetration test number of blows (SPT–N) value was calculated as the number of hammer blows for the second and third 0.5 ft intervals of penetration. SPT–N values were used to classify fine-grained sediments as very soft (< 2), soft (2–4), medium stiff (5–8), stiff (9–15), very stiff (16–30), or hard (>30) (ASTM International, 2022) and coarse-grained sediments as very loose (<4), loose (4–10), compact (10–30), and dense (>30) (Meyerhof, 1956). In very soft sediments, sampler penetration commonly required no hammer blows, and the sampler advanced under the weight of the hammer.

Penetration-test data collected during investigations in the 1990s were also included in the analysis. Although a similar method to that described by ASTM International (2022) was used, the data do not meet the ASTM International standard for calculation of SPT–N values. The 1990s penetration test results, which are reported in blow counts per foot of penetration, were collected by using a rotating cathead and rope to raise and drop a weight rather than an automatic hammer. The data are more varied but are roughly comparable to the SPT results. In this report, data collected for this study in accordance with ASTM International (2022) are recorded as SPT–N values and historical data collected during investigations in the 1990s are referred to as “hammer blows per foot.”

For depth intervals not sampled on a continuous basis, split-spoon samples were collected when the auger drilling indicated a probable change in lithology. The split-spoon samples were described in the field by texture (very fine to very coarse), sorting, color, and presence of laminations and moisture. Split-spoon samples collected from wells OD3221 and OD3226 at depths of 30 to 32 ft bls were placed in plastic bags and submitted to Kenney Geotechnical Services of Syracuse, New York, for laboratory analysis of moisture content and grain size. Two Shelby-tube samples were collected from well OD3219 at depths from 12.5 to 16 ft bls and submitted for laboratory analysis of moisture content and unconfined compressive strength in accordance with procedures and equipment described by ASTM International (2015; 2016). Very soft soils have unconfined compressive strength values less than 500 pounds per square foot (lb/ft²); soft soils have unconfined compressive strength values from 500 to 1,000 lb/ft²; and stiff soils more than 1,000 lb/ft² (Singh, 2020). Split-spoon sample descriptions and drilling notes, SPT data, and soil geotechnical properties from the wells from this study and previous studies are available from the associated USGS data release (Terry and others, 2026) and USGS GeoLog Locator (USGS, 2024a).

Test-Pit Excavation and Sediment Sampling

On May 5, 2022, test pits OD1998 and OD1999 were excavated with a backhoe to depths of 10 ft bls for field observations of sediment types and for sediment sampling in the proposed offline settling basin area (fig. 4). Sediment samples were collected in plastic bags from both test pits at a depth of 9.5 ft bls and submitted for laboratory analysis of grain size and moisture content. Sediment samples were collected in plastic bags from test pit OD1998 at a depth of 2.5 ft bls and test pit OD1999 at a depth of 5.5 ft bls and submitted for laboratory analysis of moisture content and Atterberg Limits. Atterberg Limits establish the moisture content at which fine-grained sediments transition between solid, semisolid, plastic, and liquid states (ASTM International, 2018b; Andrews and Martin, 2000; O’Kelly, 2021). Fine-grained sediments are considered liquifiable where they contain less than 15 percent clay, have a liquid limit of less than 35 percent and plasticity index (liquid limit minus plastic limit) less than 12, and their water content is more than 90 percent of the liquid limit (Seed and Idriss, 1982; Bray and Sancio, 2006; and Bennett and others, 2011). The liquidity index, which is the moisture content minus the plastic limit divided by plasticity index, can be used to classify the consistency of fine-grained sediments as very brittle (<0.00), brittle (0.00–0.25), medium soft (0.26–0.50), soft (0.51–0.75), very soft (0.76–1.00), and liquid (>1.00) (Venkatramaiah, 2006). Descriptions and geotechnical properties of the sediment samples from the test pits are available from the associated USGS data release (Terry and others, 2026).

Piezometer and Well Installation

Four piezometers and eight wells were installed as part of the study (fig. 4; table 1). The piezometers were installed by the drive-point method (Waller, 1994) along the southern reach of the proposed far east stream relocation path and provided access for groundwater level and specific conductance measurements (fig. 2). Piezometer OD3227 was installed on October 1, 2020, and piezometers OD3228, OD3229, and OD3230 were installed on November 18, 2021. The piezometers were constructed with 3/4-in diameter steel casing and 1-ft long screens.

Seven wells were installed along and near the proposed creek-relocation paths on April 11–14, 2022 (table 1). A single well (OD3219) was installed at the northwestern edge of the offline settling basin area on April 13, 2022. The wells, which provided access for geophysical logging, hydraulic-slug testing, and groundwater monitoring, were constructed with 2-in diameter polyvinyl chloride (PVC) pipe, 5- to 10-ft long screens, and as much as 10-ft long sumps. The annular space surrounding the screens were filled with sand, and the space above the screens was filled with bentonite.

Gamma and Electromagnetic-Induction Logging

Gamma and electromagnetic-induction (EMI) logs were collected from the eight wells on May 24, 2022, to provide information on grain size and groundwater salinity. The gamma and EMI logs of the wells are available in Log ASCII Standard format (LAS 2.0) from USGS GeoLog Locator (USGS, 2024a). Gamma logs measure the total gamma emission produced by naturally occurring isotopes of potassium, thorium, and uranium in sediments surrounding the well. Gamma logs were collected from the wells with a Mount Sopris Instruments gamma probe by using logging procedures described by ASTM International (2018a). Gamma activity is generally assumed to be related to grain size: gamma emissions (recorded in counts per second) increase with clay content. However, sediments that contain sand- to clay-sized grains that are dominated by quartz mineralogy with negligible clay alteration minerals, as is the case in the study area, have a reduced range of gamma response and overlap for the different grain sizes (Crow and others, 2017).

EMI logging provides a nearly continuous vertical profile of electrical conductivity of the sediments and pore water surrounding the well. The major factors affecting electrical conductivity in unconsolidated deposits is their silt and clay content, their saturated porosity, and the salinity of the groundwater. A W&R instruments three-coil induction probe that has intercoil spacings of 0.5 m and 0.8 m and operates at a frequency of 100 kilohertz was used for EMI log-data collection. The conductivity readings from the two intercoil spacings tracked each other and only that from the 0.5 m is presented. EMI logs were collected from the wells by using logging and calibration procedures as described by ASTM International (2017). The EMI probe was lowered to the bottom of the well and the probe electronics were allowed to equilibrate with the groundwater temperature (10 to 20 minutes depending on the outside air temperature). After equilibration, EMI data were collected at 0.1 ft deep intervals with the probe moving in an upward direction at a rate of 10 ft/min. Probe calibration checks were made after logging and removal of the probe from the well, during which the temperature of the probe electronics was assumed to be close to the groundwater temperature. Probe readings were recorded while holding the probe away from any metal in free air with an assumed electrical conductivity of 0 millisiemens per meter (mS/m) and while a plastic ring with an embedded wire loop that has a factory-calibrated electrical conductivity of 100 mS/m was placed at the midpoint of the intercoil spacing. Check measurements were within the accuracy range of the probe, which is ±5 mS/m.

Hydraulic-Slug Testing

Hydraulic-slug tests were completed in the eight wells on May 24, 2022, to estimate the horizontal hydraulic conductivity of the screened sediments. Slug-test design, procedures, and equipment are described by Cunningham and Schalk (2011). A Midwest Geoscience Solid H(o) Slug was used for the study tests. The slug has tapered ends to minimize the pressure wave and is designed to displace 1 ft of water in a 2-in diameter well. The slug was inserted for slug-in tests and removed for slug-out tests rapidly and smoothly. A Solinst levelogger nonvented pressure transducer with a 33-ft range was used to record water-level displacement at 1-second intervals. Three sets of manual measurements were made with a Solinst water-level meter referenced to top of casing to ensure static water-level conditions prior to slug insertion, to check transducer readings during the tests, and to determine when the tests could be ended.

The slug-test datasets were analyzed by using the Bouwer and Rice solution method (Bouwer and Rice, 1976; Bouwer, 1989) in AQTESOLV software (Duffield, 2007) to estimate hydraulic conductivity of the tested sediments. A 10:1 ratio of horizontal to vertical hydraulic conductivity was assumed for the analysis. The thickness of the tested aquifer was approximated as the difference between the top of screen depth and the water-level depth. The slug test for well OD3219 provides a representative example of the displacement data and match fit of the Bouwer and Rice analytical solution (fig. 7).

The resulting hydraulic-conductivity estimates for the wells are in table 1 and the time-displacement data are published in the associated USGS data release (Terry and others, 2026).

Monitoring of Groundwater Levels and Specific Conductance

Groundwater-level and specific-conductance data were periodically collected from the piezometers and wells after their completion. Water levels in piezometers that were above the top of casing were measured by using clear plastic tubing connected to the casing and a calibrated measuring rod. Water levels below top of casing in piezometers and wells were measured by using a Solinst water-level meter. Water samples were collected directly from the piezometers with flowing artesian conditions for field measurement of specific conductance with a YSI Incorporated conductivity meter. A bailer was used to purge and collect water samples from the wells and nonflowing piezometers for field measurement of specific conductance with a YSI Incorporated conductivity meter (USGS, variously dated).

Groundwater levels were continuously monitored in the eight wells from April 23, 2022, to April 25, 2023. Specific conductance and temperature also were continuously monitored during the same period in the screen zones of wells OD3221, OD3222, and OD3223. Solinst Levelogger sensors were used to record pressure and temperature, and Solinst Levelogger LTC sensors were used to record pressure and specific conductance. A Solinst Barologger installed in well OD1932 in the southern end of the Tully Valley was used to compensate all water-level readings for atmospheric pressure fluctuations. Pressure, barometric-pressure, and specific-conductance data were recorded at 15-minute intervals. Periodic discrete water-level measurements were made in the wells and compared with that measured by the vendor-calibrated pressure transducers. The vendor calibration of the specific conductance sensors was checked with standards from the National Water Quality Laboratory after removal from the wells and corrected using standard procedures (USGS, 2019).

Water levels measured in wells OD400, OD401, OD404, OD407, and OD408 during 1995–2010 as part of a previous investigation provided some historical long-term information on groundwater conditions in the study area, including information from an extreme flooding event in January 1996 (Lumia, 1998). The discrete and continuous groundwater-level data collected as part of previous and current investigations are available from the USGS National Water Information System (USGS, 2024b). The specific-conductance and temperature data collected as part of the current investigations are published in the associated USGS data release (Terry and others, 2026).

Proposed Creek-Relocation Paths

Four proposed creek-relocation paths were targeted for investigation, each named for its location in relation to Onondaga Creek: far east (FE), near east (NE), near west (NW), and far west (FW) (fig. 13). The shallow hydrogeologic frameworks along each of the four proposed creek relocation paths are discussed on a reach-by-reach basis (fig. 14).

Far East Path

The line of the FE path starts in the Onondaga Creek floodplain at an altitude of 570 ft, ascends the Rainbow Creek alluvial fan to an altitude of 585 ft, and descends back to the creek floodplain to an altitude of 540 ft (fig. 13).

Field observation indicates that the takeoff point for the proposed FE path from Onondaga Creek is coincident with the location where the creek overtops its banks during flooding and flood waters flow across the farm field to the ditched tributary channel (fig. 15). However, the proposed FE path takes a more easterly line from the creek and would not benefit from this natural diversion.

The first reach of the FE path (FE1) would be excavated into floodplain alluvium to a depth of 5 ft bls (figs. 8 and 16). The electrical conductivity along the reach and shear-wave velocity at seismic lines L5 and L1 indicate that the alluvium is as much as 10 ft thick, is partially to fully saturated with freshwater, consists of coarse- and fine-grained sediments, and is underlain by lacustrine fines. The shear-wave velocity at seismic line L5 indicates that the lacustrine fines are less dense than the alluvium but denser than the fines at seismic line L1, where shear-wave velocities are more typical for that type of deposit. Electrical conductivities along FE1 indicate that the shallow groundwater is fresh.

In FE1, well OD411 penetrated sand and gravel from 0 to 4.5 ft bls and massive lacustrine gray and red clay and silt from 4.5 to 57 ft bls. Well OD3224 penetrated overbank silt and clay from 0 to 2 ft bls, silty sand and gravel from 2 to 8 ft, and lacustrine clay and silt from 8 to 13 ft bls that increased in density with depth. The SPT–N values at the well indicate that the overbank fines were medium stiff and the lacustrine fines were stiff to very stiff (fig. 17A). The highest SPT–N value measured at the well site was in the lower part of the alluvium. During 2022–23, the hydraulic head measured in the well, which was screened from 3 to 8 ft bls, was influenced by the stage of the adjacent Onondaga Creek and ranged from 0.4 ft als to 2.9 ft bls. Maximum specific conductance of groundwater sampled from the well was 1,010 µS/cm. The source of the specific conductance may be due to infiltration of road-salt and (or) agricultural runoff. The estimated hydraulic conductivity of the screened alluvium was 5 feet squared per day (ft²/d).

The second reach of the proposed path (FE2) would be excavated into alluvium and lacustrine fines to a depth of 5 to 15 ft bls. (figs. 8 and 16). The electrical conductivity along the reach and shear-wave velocity at seismic lines L2 and L3 indicate the sediments are nearly fully saturated and loose near the surface and become denser with depth. Estimated electrical conductivity along FE2 may relate to increased clay content or a slight increase in salinity. Piezometer OD3227, which was installed in a circular wetland depression near FE2, penetrated organic material from 0 to 4 ft bls and clayey silt with minor fine sand from 4 to 19 ft bls. During 2022–23, the hydraulic head measured in the piezometer was from 0.8 to 7.4 ft als. The artesian flow from the piezometer was fresh. The high hydraulic head in this area may cause construction and maintenance issues during and after creek relocation. Piezometer OD3230 was installed in a wetland area and penetrated muck from 0 to 1 ft bls, silt with little clay from 1 to 5 ft bls, and gravel from 5 to 5.5 ft bls. When installed in November 2021, hydraulic head at the site was 0.40 ft bls and the specific conductance of the groundwater was 713 µS/cm.

The third reach of the FE path (FE3) would be excavated into an area with alluvium, alluvial-fan deposits, and lacustrine fines to a depth of 10 ft bls (figs. 8 and 16). This section of the electrical conductivity profile has the highest values along the proposed path and indicates the sediments are nearly fully saturated and contain brackish groundwater. As discussed in the report section “Groundwater Levels and Specific Conductance,” the source of the salinity may be upward leakage via the partially sealed salt-exploration borehole OD449 (fig. 4). Excavation below the water table would intercept groundwater and provide a continuous source of salinity to the relocated creek.

Piezometer OD3229, which was installed along reach FE3, penetrated silty clay from 0 to 2 ft bls and interbedded gravel and silt from 2 to 10 ft. During 2021–22, hydraulic heads at the site ranged from 0.75 ft als to 0.40 ft bls, and the maximum measured specific conductance of the groundwater was 2,700 µS/cm. Nearby piezometer OD3228 penetrated silty clay from 0 to 2 ft bls and silty water-bearing gravel from 2 to 2.5 ft bls. When installed in November 2021, hydraulic head at the site was above land surface, and the specific conductance of the groundwater was 2,475 µS/cm. Well OD3223 penetrated pebbly silt and clay from 0 to 5 ft bls, silty gravel from 5 to 8 ft bls, silty clay from 8 to 11 ft bls, laminated silt and clay from 11 to 27 ft bls, and very fine sand stringers at depth. The electrical conductivity log shows values (150 mS/m) from 6 to 18 ft bls, indicating the presence of brackish groundwater (fig. 17B). The SPT–N values at the well site indicate that the fines at shallow depths below land surface were soft, those at intermediate depths were stiff to very stiff, and those at the greatest depths were very soft. During 2022–23, the hydraulic head measured in well OD3223, which was screened from 7 to 17 ft bls, ranged from 0.6 to 4.2 ft bls. Maximum specific conductance of groundwater sampled from the well was 3,200 µS/cm. The estimated hydraulic conductivity of the screened sediments was 0.3 ft²/d.

The fourth reach of the FE path (FE4) would be excavated into the southern toe of the Rainbow Creek alluvial fan to a depth of about 10 ft bls (figs. 8 and 16). The electrical conductivity along the reach and shear-wave velocity at seismic line L6 indicates the presence of coarse-grained sediments that overlie lacustrine fines. These alluvial-fan deposits are unsaturated to thinly saturated with freshwater and are denser than the underlying loose lacustrine fines that become denser with depth.

The fifth reach of the FE path (FE5) would be excavated on the southern and northern slopes of the Rainbow Creek alluvial fan to a maximum depth of about 30 ft bls (figs. 8 and 16). The low electrical conductivity along the reach and high shear-wave velocity at seismic lines L7 and L9 indicated the presence of a thick sequence of relatively dense, mostly coarse-grained sediments that are unsaturated to partially saturated with freshwater. Well OD3221 penetrated clayey gravel from 0 to 7 ft bls; gravelly clay with scattered cobbles from 7 to 20 ft bls; sand, gravel, and cobbles from 20 to 36 ft bls; and dense silty clay from 36 to 37 ft bls. Sieve analysis of the split-spoon sample from 30 to 32 ft bls indicates that the sediment was a silty sand with gravel consisting of 13 percent coarse gravel, 25.4 percent fine gravel, 9.5 percent coarse sand, 15 percent medium sand, 16.1 percent fine sand, 14.9 percent silt, and 6.1 percent clay (table 2). These results are consistent with the coarse-grained nature of the alluvial-fan deposits and their observed low electrical conductivity and high shear-wave velocity The SPT–N values at the well site indicate that sediment density increased with depth from very loose to compact (fig. 17C). During 2022–23, the hydraulic head measured in the well, which was screened from 32 to 37 ft bls, ranged from 11.6 to 20.9 ft bls. During 1996–98, the hydraulic head measured in well OD 400, 420 ft upgradient from well OD3221, fluctuated almost 30 ft and rose to within 6 ft of land surface at its highest. Maximum specific conductance of groundwater sampled from well OD3221 was 720 µS/cm, which is consistent with observed electrical conductivity values. The estimated hydraulic conductivity of the screened sediments was 6 ft²/d.

Table 2.    

Laboratory analysis of sediment moisture content and compressive strength or grain size for sediment sampled from selected depths at wells OD3219, OD3221, and OD3226 and test pits OD1998 and OD1999 in the Tully Valley, Onondaga County, New York.

[Data from Terry and others (2026). Locations of wells and test pits shown on figure 4. Dates shown as month/day/year. ft NAVD 88, foot above North American Vertical Datum of 1988; ft bls, foot below land surface; MC, moisture content; %, percent; CS, compressive strength; lb/ft², pound per square foot; —, no data]

Table 2.    Laboratory analysis of sediment moisture content and compressive strength or grain size for sediment sampled from selected depths at wells OD3219, OD3221, and OD3226 and test pits OD1998 and OD1999 in the Tully Valley, Onondaga County, New York.

Site identification	Well or test pit number	Land-surface altitude (ft NAVD 88)	Type of Sample	Date of sample	Depth of sample (ft bls)	MC (%)	CS (lb/ft2)	Coarse gravel (%)	Fine gravel (%)	Coarse sand (%)	Medium sand (%)	Fine sand (%)	Silt (%)	Clay (%)	Lithology
425213076084101	OD3219	531.9	Shelby tube	4/13/2022	12.5–14	20.7	481.5	—	—	—	—	—	—	—	—
425213076084101	OD3219	531.9	Shelby tube	4/13/2022	14.5–16	23.5	905.7	—	—	—	—	—	—	—	—
425132076081001	OD3221	578.5	Spilt spoon	4/13/2022	30–32	0.1	—	13.0	25.4	9.5	15.0	16.1	14.9	6.1	Silty sand with gravel
425131076084401	OD3226	600.2	Spilt spoon	4/14/2022	30–32	14.5	—	0.0	3.9	5.4	14.6	28.5	34.5	13.1	Silty sand with clay
425211076082301	OD1998	528.8	Grab	5/12/2022	9.5	27.0	—	0.0	0.2	0.2	0.8	8.2	65.6	25.0	Silt with clay
425203076083901	OD1999	543.0	Grab	5/12/2022	9.5	32.0	—	0.0	1.6	0.8	3.0	13.8	47.7	33.1	Silty clay

Channel excavation along FE5 below the water table in the Rainbow Creek alluvial-fan deposits would intercept groundwater and make the constructed streambank susceptible to seepage-induced slope instability. The substantial water-level fluctuation in the alluvial sediments would aggravate stability conditions.

The sixth reach of the FE path (FE6) would be excavated into the northern toe of the alluvial fan and floodplain of Onondaga Creek to a depth of 5 to 10 ft bls. The electrical conductivity along the reach and shear-wave velocity at seismic line L11 indicates that the alluvial-fan deposits and alluvium are unsaturated to partially saturated with freshwater and underlain by soft lacustrine fines that become denser with depth. Well OD418 penetrated fine to medium sand from 0 to 4 ft bls, red and gray clay and silt from 4 to 30 ft bls, and massive red clay from 30 to 50 ft bls.

Near East Path

The line of the NE path starts in the Onondaga Creek floodplain at an altitude of 565 ft, crosses the toe and lower slope of the Rainbow Creek alluvial fan and reaches an altitude of 578 ft, and crosses the lower slope and toe and descending back to the creek floodplain to an altitude of 547 ft (fig. 13).

The first reach of the NE path (NE1) would be excavated into floodplain alluvium and lacustrine fines to a depth of about 5 ft bls (figs. 8 and 18). The electrical conductivity along the reach indicates that the alluvium was less than 5 ft thick, unsaturated to partially saturated, and underlain by lacustrine fines. Although the NE1 skirts the suspected mudboil area just southeast of the mudboil corridor, the electrical conductivity did not indicate brackish water (fig. 6). The shear-wave velocity at seismic lines L4 and L6 indicates that the lacustrine fines were soft at shallow depths but more compact with depth. A stream-cut exposure near NE1 shows alluvium consisting of channel sand and gravel with a buried log overlain by overbank fines (figs. 4 and 19). Well OD3222 penetrated silty clay from 0 to 6 ft bls, sandy and gravelly clay with cobbles from 6 to 15 ft bls, and laminated silt and clay from 15 to 17 ft bls. The SPT–N values at the well site indicate that the sediments were stiffer with depth (fig. 20A). During 2022–23, the hydraulic head measured in well OD3222, which was screened from 10 to 15 ft bls, ranged from 2.9 ft to 6.1 ft bls. Maximum specific conductance of groundwater sampled from the well was 1,030 µS/cm. The specific conductance may be due to infiltration of road-salt and (or) agricultural runoff. The estimated hydraulic conductivity of the screened sediments was 2 ft²/d.

The second reach of the near east path (NE2) would be excavated into the southern toe of the alluvial fan to a depth of 5 to 15 ft bls (fig. 8). The electrical conductivity along the reach indicates the presence of coarse-grained sediments that overlie lacustrine fines. These alluvial-fan deposits are unsaturated to thinly saturated with freshwater.

The third reach of the NE path (NE3) would be excavated into the southern slope, crown, and northern slope of the Rainbow Creek alluvial fan to a maximum depth of 30 ft bls. The electrical conductivity along the reach and shear-wave velocity at seismic line L7 indicate the presence of a thick sequence of relatively dense, coarse-grained sediments that are unsaturated to partially saturated with freshwater. The penetrated lithologies, penetration results, and groundwater conditions monitored at well OD3221 were described in the report section “Far East” (refer to FE5). Well OD410 penetrated road fill from 0 to 6 ft bls, sand with black organics from 6 to 8 ft bls, brown sandy gravel from 8 to 22 ft bls, silty sand and gravel with possibly some red clay layers from 22 to 26 ft bls, and brown silty sand with some gravel from 26 to 42 ft bls. The hammer-blow counts at the well site indicate that the sediments were compact between 10 and 20 ft of land surface and dense below (fig. 20B). Well OD408 penetrated sand and gravel from 0 to 52 ft bls and varved silt and clay from 52 to 55 ft bls. During 1995–2008, water levels measured in 38-ft deep well OD408 generally were between 15 to 25 ft bls. A water-level high for the well of 4.5 ft bls was measured in January 1995.

The fourth reach of the NE path (NE4) would be excavated into the toe of the Rainbow Creek alluvial fan and the Onondaga Creek floodplain to a depth of 5–10 ft bls (figs. 8 and 18). The electrical conductivity along the reach indicates that the alluvial-fan deposits and alluvium are partially saturated with freshwater. Although NE4 skirts the northeastern suspected mudboil area, the electrical conductivity gives no indication of salinity (fig. 6). Well OD409 penetrated sandy, silty, and clayey gravel from 0 to 32 ft bls and massive red clay from 32 to 69 ft bls. Well OD3220 penetrated sandy silt with some gravel and cobbles from 0 to 8 ft bls, sandy gravel from 8 to 9 ft bls, silty clay from 9 to 11 ft bls, and silty gravel with some sand and few cobbles from 11 to 22 ft bls. The hammer-blow counts and SPT–N values at the well sites indicate that the sediments were medium stiff above 15 ft bls, compact at 20 ft bls, and loose and soft below 28 ft bls (fig. 20B and C). These loose and soft sediments would provide an excavation and slope-stability challenge if they extend under the floodplain at a shallow depth.

Near West Path

The line of the NW path starts in the Onondaga Creek floodplain at an altitude of 565 ft, ascends upslope to a local high of an altitude of 580 ft, descends downslope to an altitude of 570 ft in the wetlands bordering the Main Mudboil Area, crosses the toe of the Rattlesnake Gulf creek alluvial fan, reaches an altitude of 580 ft, and descends across the mudboil corridor to the Onondaga Creek floodplain at an altitude of 547 ft (fig. 13 and 21).

The first reach of the NW path (NW1) would be excavated in floodplain alluvium to a depth of about 5 ft bls (fig. 8). The electrical conductivity profile indicates that the alluvium is partially saturated with freshwater, thins to the west, and is underlain by lacustrine fines. The second reach of the NW path (NW2) would be excavated in lacustrine fines to a depth of 5 to 10 ft bls. The electrical conductivity profile and shear-wave velocity at seismic line L18 indicates the lacustrine fines would be saturated and loose. The electrical conductivity measured in this reach is probably related to the pipeline that crosses the path.

The third reach of the NW path (NW3) skirts the Main Mudboil Area and transects a suspected mudboil area (fig. 3); NW3 would be excavated into lacustrine fines and wetland deposits to a depth of 5–15 ft bls. The electrical conductivity profile indicates the presence of brackish groundwater. Salinity is likely associated with the presence of brackish water discharged from mudboils. Excavation below the water table would intercept groundwater and provide a continuous source of salinity to the relocated creek. Wells OD79, OD412, OD405, and OD406 all penetrated 5 ft or less of sand and gravel over thick lacustrine fines. Penetration logs of wells OD405, OD406, and OD412 indicate that the lacustrine fines were very soft below a depth of 13 ft bls (fig. 22A). These very soft sediments would provide a challenge to excavation and maintenance of slope stability.

The fourth reach of the NW path (NW4) would be excavated into the toe of the Rattlesnake Gulf alluvial fan to a maximum depth of about 20 ft bls. The electrical conductivity profile and shear-wave velocity at seismic line L13 indicate the presence of about 10 ft of thinly saturated alluvial-fan deposits over loose lacustrine fines that become denser with depth. The localized anonymously high electrical conductivity is probably related to the pipeline that crosses the path and not due to the presence of brackish groundwater.

The fifth reach of the NW path (NW5) would be excavated in mudboil-corridor sediments and floodplain alluvium to a depth of 5 to 10 ft bls (fig. 8 and 21). The electrical conductivity along the reach indicates the presence of nearly fully saturated fine-grained sediments in the mudboil corridor and partially saturated alluvium in the floodplain. In the mudboil corridor, well OD434 penetrated fill from 0 to 3 ft bls; gray and red silt with some sand from 3 to 9 ft bls; mudboil-type sediments (auger flights sunk under their own weight during drilling) from 9 to 15 ft bls; soft red and gray laminated fines from 15 to 46 ft bls; and soft red and gray laminated fines with greater density and thicker laminations from 46 to 71 ft bls. The penetration log of well OD403 indicates that the sediments were soft from 6 to 18 ft bls and very soft from 18 to 48 ft bls (fig. 22D). The soft to very soft mudboil-type sediments would provide an excavation and slope stability challenge. During 1995–2010, water levels measured in 10-ft deep well OD407 generally were 2 to 4 ft bls. A water-level high for the well of 0.1 ft als was measured in June 1995. The lithology and SPT–N values of the alluvium penetrated at well OD3220 were described in the report section “Near East Path” (refer to NE4).

Far West Path

The line of the FW path starts in the Onondaga Creek floodplain at an altitude of 570 ft, ascends upslope to a local high of an altitude of 590 ft, descends downslope to an altitude of 580 ft in the floodplain of the tributary stream that flows into the Main Mudboil Area, ascends upslope and crosses the lower slope of the Rattlesnake Gulf alluvial fan to an altitude of 590 ft, and descends across the toe of the fan and to the Onondaga Creek floodplain at an altitude of 540 ft (fig. 13).

The first reach of the FW path (FW1) would be excavated into floodplain alluvium to a depth of about 5 ft bls (figs. 8 and 23). The electrical conductivity along the reach indicates that the alluvial deposits are thin, consist of coarse- and fine-grained sediments that are partially to fully saturated with freshwater, and are underlain by lacustrine fines (fig. 20). The penetrated lithologies, penetration logs, and groundwater conditions at wells OD411 and OD3224 were described in the report section “Far East Path” (refer to FE1).

The second reach of the FW path (FW2) would be excavated in alluvial-fan sand and gravel and lacustrine fines to a depth of 25 ft bls (fig. 8). The electrical conductivity profile and shear-wave velocity at seismic line L17 indicate as much as 20 ft of partially saturated, coarse-grained sediment (fig. 20). The electrical conductivity in this reach is probably related to the pipeline and not the presence of brackish groundwater. The third section of the FW path (FW3) would be excavated in alluvium and lacustrine fines to a depth of about 20 ft bls, and the fourth (FW4) would be excavated in alluvial-fan and lacustrine fines to a depth of about 30 ft bls (fig. 8). The electrical conductivity profile and shear-wave velocity at seismic lines L18 and L14 indicate a similar framework along FW3 and FW4 as present along FW2. Well OD462 penetrated 20 ft of sand and gravel above red and gray clay and silt. Well OD3225 penetrated silty sand from 0 to 2 ft bls; wet medium to coarse sand with some pebbles from 2 to 10 ft bls; and red and gray clay with silty and sandy partings from 10 to 22 ft bls. The SPT–N values at the well site indicate the sediments were soft and loose in the upper 5 ft, compact from 8 to 12 ft bls, soft at 16 ft bls, and stiff at 20 ft bls (fig. 22B). During 2022–23, the water level measured in the well, which was screened from 7 to 12 ft bls, ranged from 0.7 ft to 3.7 ft bls. Maximum specific conductance of groundwater sampled from the well was 810 µS/cm. The estimated hydraulic conductivity of the screened sediments at well OD3225 was 20 ft²/d, the second highest of the estimated values for the wells.

The fifth reach of the FW path (FW5) would be excavated into Rattlesnake Gulf alluvial-fan deposits to a depth of 10 to 20 ft bls (figs. 8 and 23). The electrical conductivity along the reach and shear-wave velocity at seismic line L15 indicates the alluvial-fan deposits are partially saturated with fresh water and consist of primarily coarse-grained sediments. The localized anomalously high electrical conductivity in this section is probably related to the pipeline and not the presence of brackish groundwater. Well OD404, located about 470 ft east and 10 ft downslope of FW5, penetrated gravelly loam from 0 to 9 ft bls; clay from 9 to 14 ft bls; sand and gravel from 14 to 24 ft bls; red clay from 24 to 26 ft bls; and clay from 26 to 28 ft bls. The penetration log at the well site indicates that the sediments increased in stiffness with depth (fig. 22D). Wells OD401 and OD402, located about 650 ft west of and 14 ft higher than FW5, penetrated gravelly silt loam from 0 to 4 ft bls; gravel from 4 to 20 ft bls; pebbly sand from 20 to 24 ft bls; sand from 24 to 27 ft bls; interbedded sand and red clay from 27 to 43 ft bls; and “heaving” sand from 43 to 50 ft bls. “Heaving” sand is fine sand that flows into the augers under artesian pressure (Hackett, 1987). Nearby well OD3226 penetrated sand and gravel with clay and some cobbles from 0 to 20 ft bls; sand and gravel with clay, some cobbles, and sand and silt beds from 20 to 36 ft bls; and silty clay from 36 to 37 ft bls. Sieve analysis of the split-spoon sample from 30 to 32 ft bls indicates that the sediment was a silty sand with clay consisting of 3.9 percent fine gravel, 5.4 percent coarse sand, 14.6 percent medium sand, 28.5 percent fine sand, 34.5 percent silt, and 13.1 percent clay (table 2). The penetration logs at wells OD402 and OD3226 indicate the sediments were soft to very soft below a depth of 35 ft (fig. 22C and D). During 2022–23, the water level measured in well OD3226, which was screened from 31 to 36 ft bls, ranged from 22.5 to 25.0 ft bls. During 1996–2009, the hydraulic heads measured in 23-ft deep well OD404 and 40-ft deep well OD401 fluctuated more than 15 ft and rose to within 7 ft of land surface. Maximum specific conductance of groundwater sampled from the well OD3226 was 780 µS/cm, and the estimated hydraulic conductivity of the screened sediments was 0.5 ft²/d.

Channel excavation along FW4 and FW5 below the water table in the Rattlesnake Gulf alluvial-fan deposits would intercept groundwater and make the constructed streambank susceptible to seepage-induced slope instability. The substantial water-level fluctuation would aggravate the stability condition. In addition, excavation along FE5 would have the potential of intercepting groundwater that currently discharges to water-supply springs at the toe of the Rattlesnake Gulf alluvial fan just north of Otisco Road and west of Onondaga Creek (fig. 4).

The sixth reach of the FW path (FW6) crosses the toe of the Rattlesnake Gulf alluvial fan and Onondaga Creek floodplain and would be excavated into fan and floodplain alluvium and lacustrine fines to a depth of 5 to 10 ft bls (figs. 8 and 23). The electrical conductivity along the reach and shear-wave velocity at seismic line L12 indicates that the alluvium is partially saturated with fresh water and consists of coarse- and fine-grained sediments. The observed electrical conductivity in this section is probably related to the pipeline and not the presence of brackish groundwater. Well OD418 penetrated fine to medium sand from 0 to 4 ft bls; red and gray clay and silt from 4 to 30 ft bls; and massive red clay from 30 to 50 ft bls.

Proposed Offline Settling Basin Area

The proposed offline settling basin area is in the northern part of the Rattlesnake Gulf alluvial fan and would be about 4,500 ft long and 1,300 ft wide (figs. 2 and 8). The center of this rectangular area is about 3,300 ft north of Otisco Road, 1,700 ft south of Nichols Road, 1,700 ft east of Tully Farms Road, and 1,600 ft west of Onondaga Creek and New York State Route 11A. The southeastern corner of the area is about 1,500 ft north of the mudboil corridor and 500 ft west of where Rattlesnake Gulf and Rainbow Creek flow into Onondaga Creek.

Natural and human-made diversions of Rattlesnake Gulf have resulted in saturated conditions in the general area of the proposed offline settling basin (fig. 24). The Rattlesnake Gulf channel has aggraded during flooding raising the channel about 3 ft and causing the creek to flow off the aggraded channel to the south and north. The northern diversion flows along the southern and then eastern edge of a farm field and flows north to the area of the proposed settling basins. A human-made diversion of this flow channels the water toward Onondaga Creek just north of the proposed basin area, but varied topography allows some of the water to flow in an easterly direction across this area prior to this diversion. A recent attempt to rechannel Rattlesnake Gulf creek on the valley floor might help reduce the formerly diverted flow from the creek, but the aggradation process will likely continue with sand and gravel infilling the channel, and the flow returning to the north and south diversion channels unless the newly excavated channel is maintained.

The proposed offline settling basin would be excavated in, and the berms would be constructed on, alluvial-fan deposits and lacustrine fines. The electrical conductivity profile indicates that alluvial deposits are thick and saturated with freshwater below about 10 ft bls near Nichols Road (fig. 25). To the northeast of the section by Nichols Road, well OD435 penetrated brown alluvial gravel from 0 to 30 ft bls and massive red clay grading to silt from 30 to 104 ft bls. Eight split-spoon samples of the lacustrine fines collected from 30 to 46.9 ft bls had moisture contents from 26.8 to 44.6 percent, liquid limits from 28.8 to 45.3 percent, plastic limits from 16.2 to 27.4 percent, plasticity indexes from 10.5 to 23.4, and liquidity indexes from 0.8 to 1.2 (table 3).

The thickness of the alluvial deposits decreases to the south, and at the northern border of the proposed area, there is less than 10 ft of sand and gravel overlying the lacustrine fines. Well OD3219 at the northwestern corner of the proposed area penetrated sand and gravel with silt from 0 to 7 ft bls; silt with many sand stringers from 7 to 10 ft bls; and massive silt with some sand and gravel stringers from 10 to 17 ft bls. The SPT–N values at the well site indicate that the lacustrine fines were soft to very soft (fig. 26). The unconfined compressive strength of the sediments at 12.5 to 14 ft bls was 481.5 lb/ft² and that at 14.5 to 16 ft bls was 905.7 lb/ft² indicating very soft to soft soils. During 2022–23, the hydraulic head measured in the well, which was screened from 2 to 7 ft bls, ranged from 0.9 ft als to 1.5 bls. Maximum specific conductance of groundwater sampled from the well was 680 µS/cm. The estimated hydraulic conductivity of the screened sediments was 10 ft²/d.

The electrical conductivity along the western border of the proposed basin indicates that the alluvial deposits become thinner and finer grained between well OD3219 and test pit OD1999, and that the maximum depth to freshwater saturation is about 5 ft bls. Test pit OD1999 was excavated into topsoil from 0 to 2 ft bls; brown silty clay with small pebbles from 2 to 4 ft bls; wet brown silty clay with rounded pebbles and cobbles from 4 to 5 ft bls; gray silty clay from 5 to 7 ft bls; gray silty clay with trace pebbles from 7 to 9 ft bls; and gray clayey silt with very fine to fine sand (appears to be liquefiable) from 9 to 10 ft bls (fig. 27). A grab sample of the lacustrine fines from 5.5 ft bls had a moisture content of 52.3 percent, liquid limit of 51 percent, plastic limit of 32 percent, plasticity index of 19, and liquidity index of 1.1 (table 3).

The electrical conductivity along the middle part of the proposed area from test pit OD1999 to test pit OD1998 indicates (1) the presence of only very thin alluvial deposits, which are primarily fine grained, overlying the lacustrine fines and (2) the depth to freshwater saturation shallows from west to east. The localized electrical conductivity in this section is probably related to the pipeline and not due to the presence of brackish groundwater. Shear-wave velocities determined from seismic lines L21, L19, and L20 are consistent with the fine-grained character of the sediments. Test pit OD1999 was excavated into topsoil from 0 to 1 ft bls; brown clay from 1 to 6 ft bls; saturated gray silty clay and gravel with rounded small pebbles from 6 to 7 ft bls; gray silty clay with trace pebbles from 7 to 9 ft bls; and gray clayey silt with very fine to fine sand (appears liquefiable) from 9 to 10 ft bls (fig. 25). Grab samples of the fines from 2.5 ft bls had a moisture content of 17.5 percent, liquid limit of 34 percent, plastic limit of 22 percent, and plasticity index of 12. Well OD419 near New York State Route 11A penetrated silt from 0 to 7.5 ft bls; clay from 7.5 to 13.5 ft bls; silt from 13.5 to 19.5 ft bls; sand from 19.5 to 25 ft bls; gravel from 25 to 26.5 ft bls; sand from 26.5 to 31.5 ft bls; silt from 31.5 to 36 ft bls; and grayish bedrock from 36 to 42 ft bls. The well was difficult to grout up when it was decommissioned because of flowing artesian conditions.

Excavation, berm construction, and loading of the saturated, soft to very soft deposits underlying the proposed offline settling basin area may be problematic. Soil strengthening may be needed during the construction of the proposed basin.