Abstract

Domagalski, J. and Phillips, S. 2006, Transport of Agricultural Chemicals: Unsaturated Zone to Ground Water to Surface Water, San Joaquin Valley, California, National Water Quality Monitoring Council, 5^th National Monitoring Conference, May 7-11, 2006, San Jose, CA. 

Transport of agricultural chemicals (nutrients and selected pesticides) was examined along a 1-kilometer flow path from an almond orchard to the Merced River in the San Joaquin Valley, California, as part of a national study on the fate of these chemicals within the hydrological cycle of small watersheds. Fruit and nut orchards represent a major agricultural commodity in the valley, and interest has grown regarding the management of these fields to prevent degradation of water resources. The soil and aquifer mineralogy in this location is dominated by granitic sand (derived from the adjacent Sierra Nevada) with low amounts of organic carbon. Nitrogen application rates of 200 kg/hectare per year result in nitrate concentrations in the unsaturated zone (approximate thickness of 6.5 meters) of up to 40 mg/L as N, and concentrations in ground water exceeding the drinking water standard of 10 mg/L to a depth of 30 meters below land surface. Redox conditions along most of the flow path are consistent with low dissolved oxygen and manganese reduction, and dissolved gases show excess nitrogen indicating nitrate reduction is taking place. The age distribution of water along this flow path is from 6 to 20 years. The redox chemistry changes substantially across the riparian buffer between the orchards and the river. Within the riparian part of the flow path, there is evidence of iron and sulfate reduction, and nitrate concentrations decrease to 1 mg/L or less in many locations. The riparian buffer is an apparent effective environment for removing nitrate from ground water. The flow path terminates at the riverbed, with seasonal groundwater discharge into the river, depending on the river stage. Unlike nutrients, few pesticides were detected in either the unsaturated or saturated zone. Simazine was the most frequently detected, and was present in all water younger than 20 years