By Loren F. Metzger
U.S. GEOLOGICAL SURVEY
Water-Resources Investigations Report 02-4078
Sacramento, California 2002
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San Francisquito Creek is an important source of recharge to the 22-square-mile San Francisquito Creek alluvial fan ground-water subbasin in the southern San Mateo and northern Santa Clara Counties of California. Ground water supplies as much as 20 percent of the water to some area communities. Local residents are concerned that infiltration and consequently ground-water recharge would be reduced if additional flood-control measures are implemented along San Francisquito Creek. To improve the understanding of the surface-water/ground-water interaction between San Francisquito Creek and the San Francisquito Creek alluvial fan, the U.S. Geological Survey (USGS) estimated streamflow gains and losses along San Francisquito Creek and determined the chemical quality and isotopic composition of surface and ground water in the study area.
Streamflow was measured at 13 temporary streamflow-measurement stations to determine streamflow gains and losses along a 8.4-mile section of San Francisquito Creek. A series of five seepage runs between April 1996 and May 1997 indicate that losses in San Francisquito Creek were negligible until it crossed the Pulgas Fault at Sand Hill Road. Streamflow losses increased between Sand Hill Road and Middlefield Road where the alluvial deposits are predominantly coarse-grained and the water table is below the bottom of the channel. The greatest streamflow losses were measured along a 1.8-mile section of the creek between the San Mateo Drive bike bridge and Middlefield Road; average losses between San Mateo Drive and Alma Street and from there to Middlefield Road were 3.1 and 2.5 acre-feet per day, respectively.
Downstream from Middlefield Road, streamflow gains and losses owing to seepage may be masked by urban runoff, changes in bank storage, and tidal effects from San Francisco Bay. Streamflow gains measured between Middlefield Road and the 1200 block of Woodland Avenue may be attributable to urban runoff and (or) ground-water inflow. Water-level measurements from nearby wells indicate that the regional water table may coincide with the channel bottom along this reach of San Francisquito Creek, particularly during the winter and early spring when water levels usually reach their maximum. Streamflow losses resumed below the 1200 block of Woodland Avenue, extending downstream to Newell Road. Discharge from a large storm drain between Newell Road and East Bayshore Road may account for the streamflow gains measured between these sites. Streamflow gains were measured between East Bayshore Road and the Palo Alto Municipal Golf Course, but this reach is difficult to characterize because of the probable influence of high tides.
Estimated average streamflow losses totaled approximately 1,050 acre-feet per year for the reaches between USGS stream gage 11164500 at Stanford University (upstream of Junipero Serra Boulevard) and the Palo Alto Municipal Golf Course, including approximately 595 acre-feet per year for the 1.8-mile section between San Mateo Drive and Middlefield Road. Approximately 58 percent, or 550 acre-feet, of the total estimated average annual recharge from San Francisquito Creek occurs between the San Mateo Drive and Middlefield Road sites.
The chemical composition of San Francisquito Creek water varies as a function of seasonal changes in hydrologic conditions. Measurements of specific conductance indicate that during dry weather and low flow, the dissolved-solids concentrations tends to be high, and during wet weather, the concentration tends to be low owing to dilution by surface water. Compared with water samples from upstream sites at USGS stream gage 11164500 and San Mateo Drive, the samples from the downstream sites at Alma Street and Woodland Avenue had low specific conductance; low concentrations of magnesium, sodium, sulfate, chloride, boron, and total dissolved solids; high nutrient concentrations; and light isotopic compositions indicating that urban runoff constitutes most of the streamflow in some reaches during low flow.
The chemical composition of ground water in the study area varies primarily as a function of aquifer depth, changing from a calcium-bicarbonate or mixed cation-bicarbonate water in the shallow aquifer to a sodium-chloride or mixed cation-mixed anion water in the lower zone of the deep aquifer. The most pronounced difference in ground-water composition between the shallow and deep aquifers occurs in the lower part of the San Francisquito Creek alluvial fan downstream of Alma Street owing to extensive deposits of bay mud and clay separating the two aquifers. The concentration of chloride in samples from some of the wells exceeds 100 milligrams per liter. Ratios of selected trace elements to chloride indicate that modern bay water intrusion is not the source of the high chloride concentrations: water moving through the deep aquifer may reach chloride-rich marine sediments where mineral dissolution may increase the concentrations of sodium and chloride.
Isotopic ratios of oxygen and hydrogen in water from selected surface-water sites, public supply, and selected production wells plot below, but parallel to, the global meteoric water line. The isotopically heaviest water was from Lake Lagunita and the isotopically lightest water was imported public supply water. With the exception of isotope samples collected from San Francisquito Creek at Alma Street and the 1200 block of Woodland Avenue during low-flow conditions, stream samples were isotopically heavier than ground-water samples. The isotopically heaviest ground-water samples were from wells near losing reaches of San Francisquito Creek. The isotopically lightest samples were from wells completed in the shallow aquifer and located close to residential streets. Water to these wells may be a mixture of native ground-water and imported water from leaking public water supply and sewage lines and return flow from excess irrigation of landscaping. The isotopic data also indicate that bay water intrusion is not the source of the high chloride concentrations in water from the wells sampled for this study.
Measured Streamflow Gains and Losses
Methods of Data Collection and Analysis
Streamflow Measurements and Estimated Gains and Losses
Estimated Ground-Water Recharge from San Francisquito Creek
Characterization of Surface-Water and Ground-Water Quality
Methods of Water Sampling and Analysis
Oxygen-18 and Deuterium
Summary and Conclusions
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