HR: 1340h
AN: H43B-0373    [Abstracts]
TI: Denitrification in a Shallow Aquifer Underlying a Dairy Farm in the Central Valley of California
AU: * Esser, B K
EM: esser1@llnl.gov
AF: Chemical, Biology, and Nuclear Science Division, L-231, Lawrence Livermore National Laboratory, PO Box 808, Livermore, CA 94551 United States
AU: Beller, H R
EM: beller2@llnl.gov
AF: Environmental Restoration Division, L-542, Lawrence Livermore National Laboratory, PO Box 808, Livermore, CA 94551 United States
AU: Carle, S F
EM: carle1@llnl.gov
AF: Environmental Sciences Division, L-208, Lawrence Livermore National Laboratory, PO Box 808, Livermore, CA 94551 United States
AU: Hudson, G B
EM: hudson5@llnl.gov
AF: Chemical, Biology, and Nuclear Science Division, L-231, Lawrence Livermore National Laboratory, PO Box 808, Livermore, CA 94551 United States
AU: Kane, S R
EM: kane11@llnl.gov
AF: Environmental Restoration Division, L-542, Lawrence Livermore National Laboratory, PO Box 808, Livermore, CA 94551 United States
AU: Mcnab, W W
EM: mcnab1@llnl.gov
AF: Environmental Restoration Division, L-542, Lawrence Livermore National Laboratory, PO Box 808, Livermore, CA 94551 United States
AU: Moran, J E
EM: moran10@llnl.gov
AF: Chemical, Biology, and Nuclear Science Division, L-231, Lawrence Livermore National Laboratory, PO Box 808, Livermore, CA 94551 United States
AU: Tompson, A F
EM: afbt@llnl.gov
AF: Environmental Sciences Division, L-208, Lawrence Livermore National Laboratory, PO Box 808, Livermore, CA 94551 United States
AB: Nitrate loading to shallow aquifers from dairy farm operations presents a serious threat to critical groundwater resources in California. Less well known is the extent to which saturated zone denitrification may mitigate the problem by converting nitrate to the benign end-product nitrogen, before nitrate is transported to deeper aquifers used for drinking water. We are carrying out a multi-disciplinary study of saturated zone denitrification, in a dense network of monitoring points at a 1500-cow dairy in Kings County, California. Detailed vertical profiles of anion and cation concentrations, along with dissolved excess nitrogen were obtained at five-foot intervals using temporary direct-push wells. Results show nitrate concentrations in excess of 100 mg/L over the top few meters of the water column, abruptly falling to less than 5 mg/L below a depth of approximately 10m. Over the same interval, dissolved excess nitrogen concentrations sharply increase, indicating that denitrification is responsible for a significant fraction of the nitrate decrease. This pattern is in effect across the entire dairy site. A key aspect of the project is a concurrent focus on understanding the hydrogeology of the site. Regionally, overdraft over the past several decades has resulted in the development of separate shallow (10 m) and deeper ($\ge$ 40 m) aquifer systems. Recharge to the shallow aquifer is derived from low TDS, isotopically depleted Kings River water from a nearby unlined irrigation canal. Local agricultural pumping from the shallow aquifer and infiltration from irrigation water are significant factors in the shallow system. The deeper aquifer is characterized by intensive regional pumping, rapidly decreasing water levels, and the apparent disposition of the shallow aquifer as a perched system for a 1-km$^2$ or more area surrounding the farm. The air gap separating the aquifers is low in oxygen and undergoes pressure changes as water levels fluctuate below. Age dating and negligible nitrate levels in the lower aquifer suggest a long horizontal recharge pathway from the perimeter of the perched area, while sustainability of the shallow system is dependent upon continuing recharge through the irrigation canal. This work was conducted under the auspices of the U.S. Department of Energy by the University of California, Lawrence Livermore National Laboratory under contract W-7405-Eng-48.
DE: 6344 System operation and management
DE: 1829 Groundwater hydrology
DE: 1831 Groundwater quality
DE: 1832 Groundwater transport
DE: 1884 Water supply
SC: Hydrology [H]
MN: 2004 AGU Fall Meeting