USGS Groundwater Information
Nitrate contamination and incorporation of excess air associated with artificial recharge in a desert basin, Yucca Valley, California
By Jill N. Densmore1 and J.K. Böhlke2
Historically, ground water has been the the sole source of water supply for the town of Yucca Valley in the Mojave Desert of southern California and, due to pumping, ground-water levels have declined as much as 300 feet since the 1950's (fig. 1). To reverse declines and to ensure future water needs are met, in 1995, the Hi-Desert Water District began recharging the basin using water imported from northern California. Subsequently, water levels rose as much as 240 feet (1999), and nitrate concentrations increased, in some cases, exceeding maximum contaminant levels established by the U.S. Environmental Protection Agency. The high nitrate content water also contains large quantities of dissolved gas, which when out-gassed in the delivery system, causes cloudy water and air accumulation within pipes.
In 1993, two multiple-completion monitoring sites (YV1 and YV2) were drilled near two artificial recharge ponds in the center of the basin. These sites were designed to monitor rising ground-water levels expected from the artificial recharge. In addition, water-quality samples were collected from these sites and from other existing well sites prior to, during, and after recharge to monitor changes in ground-water quality. The water samples were analyzed for major ions, nutrients, selected trace elements, and stable isotopes of hydrogen and oxygen (δ2H and δ18O). Selected ground-water samples were analyzed for nitrogen isotopes (δ15N) to determine the source of increased nitrate, and for dissolved gases (nitrogen, argon, carbon dioxide, and oxygen) to determine the source of excess gas and to evaluate the possibility of denitrification resulting in an increase in the concentration of dissolved N2.
Nitrate concentrations in production well 17E (perforated from 450 to 800 ft below land surface and located near the west recharge pond) have increased from 20 mg/L as nitrate (NO3) in 1995 to 82 mg/L in 1999. In contrast, during this same period, nitrate concentrations in an adjacent deeper well (16E, perforated from 920 to 1,450 ft below land surface) have remained constant at 10 mg/L as NO3 and represent background levels. The nitrate concentration in well 17E, in January 1995, which was already higher than background levels, suggests that nitrate from surficial sources may have reached the shallow wells in some parts of the basin before recharge of imported water began. Because the nitrate concentration of imported water is low (0.6 mg/L), it is not likely the source of high nitrate concentrations in wells near the percolation ponds. δ15N values indicate that the high nitrate concentrations in wells near the ponds could result in part, from septic-tank seepage in the unsaturated zone. In Yucca Valley, wastewater from homes and businesses is disposed of in septic tanks.
Dissolved nitrogen (N2)gas concentrations ranged from 19 to 51 mg/L, argon (Ar) concentrations ranged from 0.6 to 1.3 mg/L, and oxygen (O2) concentrations ranged from 6 to 19 mg/L; some of these greatly exceed air-saturation values. The large variations in dissolved gas concentrations indicate that varying amounts of excess air were trapped in the water during artificial recharge through a thick unsaturated zone [more than 300 feet in 1995 (prior to artificial recharge)] and at least partly dissolved below the water table. Samples with the highest dissolved gas concentrations (from production wells 7E and 12E, which are adjacent to the west pond) had almost twice the concentrations of Ar and N2 expected from air-water equilibrium, equivalent to about 25-30 ccSTP/L of excess air. Other samples having high gas concentrations were from production wells 14E and 17E (near the east and west ponds, respectively, had about 17-20 ccSTP/L of excess air) and from YV1-305' and YV1- 230' (adjacent to the east pond had about 6-10 ccSTP/L of excess air). In contrast, the samples from production wells 9W and 8W, in the western part of the basin had the equivalent of about 3-6 ccSTP/L of excess air, with apparent recharge equilibration temperatures of 10-13°C. These samples are from outside the area of artificial recharge and the values are more typical of the natural recharge process.
This study indicates unanticipated water-quality problems have resulted from artificial recharge efforts in Yucca Valley. It is proposed that increases in nitrate concentrations are due to the interception of downward-moving septic seepage by the rising water table and perhaps from the incorporation of natural pore-water nitrate from the unsaturated zone. In addition, large amounts of excess air were incorporated in the ground water due to artificial recharge through a thick unsaturated zone, resulting in undesired out-gassing in the delivery system causing cloudy water, air accumulation within pipes, and customer complaints.
In George R. Aiken and Eve L. Kuniansky, editors, 2002, U.S. Geological Survey Artificial Recharge Workshop Proceedings, Sacramento, California, April 2-4, 2002: USGS Open-File Report 02-89
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