Submitted to: American Geophysical Union
Publication Type: Abstract Only
Publication Acceptance Date: 12/15/1995
Publication Date: N/A
Technical Abstract: Alluvial aquifers have been shown to be among the most susceptible to agrichemical contamination. Leaching of chemicals by precipitation or irrigation water through soils is the most frequently studied process for such contamination. However, preliminary measurements show substantial infiltration of agrichemicals through a tributary stream bed and the process warrants further investigation. Infiltration occurred through the bed of Walnut Creek as it traverses the flood plain and an underlying alluvial aquifer. Infiltration was estimated by measuring the loss of stream discharge in Walnut Creek and the concentrations of agrichemicals in shallow groundwater beneath the stream bed. Infiltration was estimated before application of chemicals to the fields during a dry period in April 1994, and after application of agrichemicals during a small runoff event in June. In April, the flux of herbicides and metabolites through the creek bed ranged from less than 10 to 270 ug/d/m**2. In June, the flux of herbicides and metabolites was from 70 to 3060 ug/d/m**2. The flux rate of nitrate-nitrogen was greater than 2,000 mg/d/m**2 for both sampling periods. The fluxes of atrazine beneath Walnut Creek were 2 to 5 orders of magnitude greater than the estimated flux of atrazine caused by leaching through a field. Also, the flux of water from Walnut Creek to alluvial groundwater was about three orders of magnitude greater than estimated recharge from precipitation. The large flux of agrichemicals from Walnut Creek to alluvial groundwater is due in part to the conductive sands and rather fast groundwater velocities. Average vertical hydraulic conductivity through the stream bed was calculated as 30 to 90 m/d, and estimated groundwater velocities were from 1 to 5 m/d.