Open-File Report 2006-1377: Results

On the seismic-reflection sections, we identify four seismic-stratigraphic units (Figure 4), the first three of which we mapped to obtain sediment extent and thickness (Figures 3A-E). Seismic sections illustrate the various units in different parts of the region (Figures 4-6).

Units 1, 2, and 3. Units 1 and 2 (Figures 4 and 5) can only be separated in the eastern sediment accumulation of Spring Creek Arm; elsewhere the units are mapped simply as Unit 1. Unit 1 is almost seismically transparent and it directly underlies the reservoir floor. In the Spring Creek Arm, reflections from within the unit are parallel to each other and to the water-bottom, dip into the thalweg of the arm or toward the arm entrance, and onlap the underlying units. Reflections are present within the unit, but can barely be seen on the Geopulse records. This lack of reflectivity indicates that only small contrasts in density and velocity characterize the sedimentary layers within the unit, that the unit was almost the same density and velocity as the overlying water, and was highly unconsolidated. The top of the unit was often difficult to determine on the Geopulse records, but is clearly seen on the 200-kHz bathymetric records. Based on sampling observations, the unit top is virtually formed by suspended sediment in the water column, leading to the low reflectivity of the surface.

Unit 2 has the same geometry as Unit 1, except that toward the mouth of the arm the unit reflectors are mounded instead of parallel with the water bottom or the overlying Unit 1 reflections. This mounding could indicate either current disruption within the unit or possibly slumping toward the main reservoir. Internal reflections within Unit 2 are stronger than in Unit 1, and the boundary between the units is where the strong reflections begin. The greater reflectivity within Unit 2 probably indicates a greater degree of compaction of Unit 2 than in Unit 1.

Unit 3 is a highly reflective, wedge-shaped unit that onlaps the underlying bedrock and alluvial sediments. It also thins toward the thalweg of the arm and toward the main reservoir (Figures 4 and 5). The high reflectivity of the unit contrasts markedly with the low reflectivity in Units 1 and 2, and indicates that Unit 3 was probably moderately well lithified. The unit was present in both the central and eastern fine-grained sediment accumulations of Spring Creek Arm; we have not interpreted its presence in the western Spring Creek Arm or in the main Keswick Reservoir.

During 1993-1994, we initially interpreted these data to mean that Unit 3 was a remnant of the debris-delta that was deposited into the arm prior to construction of the Spring Creek Debris Dam. This interpretation was based on comparison of cross-sections of the seismic data with cross-sections of the debris fan given by Prokopovich (1991), and because Unit 3 generally corresponded to the debris fan. We proposed that the unit was probably composed of well-lithified material or alluvial material laid down in the debris fan, and did not include it in our first estimate of precipitated acid-mine drainage sediment (the initial estimate was about 110,000 cubic meters of sediment in the three sediment accumulations in the Spring Creek Arm). Subsequent sampling has shown that Unit 3 material is virtually indistinguishable in chemical composition from the overlying units, although it has a lower water content (Nordstrom and others, 1999). The revised volumes for the sediment accumulations given herein reflect the added volume of this unit.

Sediment sampling shows that all three units are dominantly (>90%) composed of chemically precipitated material. Some thin (<1 cm) sand layers are present in cores, probably deposited during major storm and overflow events from the Spring Creek Debris Dam. The sediment in Keswick Reservoir has been sampled in the old Sacramento River channel near the Spring Creek Arm, and consists dominantly of precipitated sediment, although there are more and thicker sand layers.

Unit 4. We have not tried to differentiate seismic sequences below Unit 3, but have included all such sequences into Unit 4. In some areas (for example, beneath the western part of Spring Creek Arm and below much of Keswick Reservoir) the top of Unit 4 is acoustic basement and is probably composed of the Paleozoic and Mesozoic metamorphic and igneous rocks that underlie the region. Near the mouth of Spring Creek Arm and below the old Sacramento River channel, sequences of layered strata were present. These sequences probably consist primarily of alluvial deposits emplaced prior to construction of Keswick Dam, but could also include fine-grained sediment and mining debris and smelter slag.

Seismic Line 32, Spring Creek Arm (Figure 4). On this north-south crossing of Spring Creek Arm, Units 1, 2, and 3 dip and thin northward, terminating adjacent to the north wall of the arm. The top of Unit 1 is barely evident on the north end of the line, and cannot be followed in the uppermost part of the record; however the top is visible on concurrently acquired bathymetric records. The high-amplitude reflector separating Units 1 and 2 is particularly visible, as is the highly reflective wedge of Unit 3. Unit 4 is layered and probably consists of alluvial sequences. The thalweb channel in the arm carries water discharged from the Spring Creek Power Plant, and crosses through the arm at navigation times between about 23:07 and 23:07:30 (annotated at the top of the seismic section), thus keeping this part of the arm relatively free of acid-mine drainage sediment. The deposition of Units 1-3 was controlled primarily by the interplay of sediment accumulating in back eddies in the arm, and by current flow eroding these sediment banks adjacent to the main current channel.

Seismic Line 53, Spring Creek Arm (Figure 5). This line shows the eastward-building delta prograding out of the arm into the main reservoir. Units 1 and 2 are easily separated, with a marked increase in reflectivity in Unit 2, and with mounding of the unit at the eastern end of the prograding delta. Unit 3 also thins eastward beneath the arm. A submerged railroad bridge is present at the mouth of Spring Creek Arm; on the unmigrated seismic section, this bridge shows as a nested pair of hyperbolic reflections on the eastern end of the record.

Seismic Line 37, Old Sacramento River Channel adjacent to Spring Creek Arm (Figure 6). A seismically transparent layer thicker than 3 m had accumulated within the old (pre-reservoir) bed of the Sacramento River. We assign this to Unit 1 based on both the seismic character and on sediment samples from the unit. Beneath the center of the channel, layered reflections could be acid-mine drainage sediments, but more probably these reflections are from alluvial accumulations in the old riverbed. Elsewhere, the relatively chaotic reflections indicate very coarse (boulder and cobble) alluvial material or Paleozoic and Mesozoic bedrock.

We mapped seismic Units 1, 2, and 3 on the seismic-reflection data, converted reflection times to depth using water velocity (1500 m/s), and mapped the resulting thickness of the chemically precipitated fine-grained sediment resulting from mixing of acid-mine water with neutral pH reservoir water (Figures 3A-E). The sediment-thickness map shows three areas of sediment accumulation in Spring Creek Arm, termed here the western, central, and eastern sediment accumulations. When water from the Spring Creek Power Plant moves through the arm, each of the sediment accumulations lies below a back eddy in the current flow.

Our knowledge of the distribution of sediment in the western accumulation (Figure 3B-2) is informed by only a small amount of seismic-reflection data; the extent of the mapped sediment bank is inferred from an aerial photograph of the arm. The thickness shown is measured from the seismic data; however, sediment was demonstrably thicker than shown based on two cores recovered in August 1995 that were 2.1 and 2.75 m long.

The central sediment accumulation was greater than 5 m thick. Seismic mapping shows a maximum accumulation area near the north bank of the arm, thinning southward into the Spring Creek Arm thalweg.

The eastern sediment accumulation was more than 8 m thick and thinned northward and eastward into the Spring Creek current channel and into the main reservoir. Deposition of this sediment body may be partly controlled by the location of the submerged railroad bridge; sediment has apparently backed up behind the south side of the bridge and its abutments. The current along the arm hugs the north bank and follows the old Spring Creek channel beneath the bridge. On aerial photographs and in the reservoir, water from the arm (and entrained sediments) moves around the corner of the arm southward into the reservoir along a submerged railroad cut just south of the bridge.

In the main reservoir, a thin Unit 1 layer can be mapped northward of Spring Creek Arm within the old river channel; this unit covers much of the bottom of the reservoir adjacent to the arm. The sediment reached a maximum thickness greater than 3 m in the old river channel opposite the arm and adjacent to the steep east bank of Keswick Reservoir. The sediment thinned downstream to less than 1 m thick near Keswick Dam, with the thickest sediment confined to the old river channel. Sediment was present outside of the river channel, however, and thinly covered most of the flat submerged river bottom. Near Keswick Dam, the fine-grained sediment was mainly confined to the old river channel. We could not survey past the safety boom near Keswick Dam, and therefore do not know the sediment distribution near or at the base of the dam.

Based on the contours interpreted for fine-grained sediment thickness, we estimate that the total volume of fine-grained sediment in the Spring Creek Arm of Keswick Reservoir precipitated from acid-mine drainage was about 152,000 cubic meters, with 14,000, 32,000, and 105,000 cubic meters respectively in the western, central, and eastern accumulations. We estimate that an additional 110, 000 cubic meters of material was present in the main channel of Keswick Reservoir. The total volume of potentially contaminated, fine-grained sediments in Keswick Reservoir is therefore estimated to have been about 260,000 cubic meters in February 1993.

In the Spring Creek Arm, probably the entire bottom was covered by or composed of acid-mine drainage sediment with the exception of the thalweg profile. Areas outside of the fine-grained sediment accumulations probably have a bottom composed of combinations of slag, old delta debris deposited before construction of the Spring Creek Debris Dam, or lithified sediment laid down in current-swept areas.

In the main channel of Keswick Reservoir, much of the reservoir bottom downstream of the Spring Creek Arm confluence appears to have been covered by acid-mine drainage sediment. In the areas where sediment was mapped using high-resolution seismic data, the percent of the bottom covered is as follows:

Lower main reservoir near dam to log boom near dam (Figure 3E, 40°37'06" to log boom): 12% covered, mainly in the former Sacramento River channel. No information is available between the log boom and Keswick Dam.

Overall, the average area of the main channel from Spring Creek Arm to Keswick Dam covered with mapped, fine-grained sediment was about 29%.

Substantially larger areas of the bottom could have a thin acid-mine drainage sediment cover that is not mappable using the seismic-reflection data.

Distribution, thickness, and volume of fine-grained sediment from precipitation of metals from acid-mine waters in Keswick Reservoir, Shasta County, California

Results