The High Plains aquifer underlies 450,000 km² of the semiarid western United States and supplies about 30 percent of the ground water used for irrigation in the country. Detections of elevated concentrations of nitrate and atrazine in ground water and 30 to 60 years of irrigated agriculture in the region indicate irrigation may be a key factor influencing chemical transport to the water table. However, because of the relatively thick unsaturated zone overlying the aquifer, the processes that transport water and chemicals to the water table are not well understood. In 2000, the average depth to ground water in the High Plains aquifer was 30 m. Tritium profiles in the unsaturated zone were used to evaluate water fluxes at rangeland and irrigated sites in Kansas and Texas. Nitrate and atrazine concentrations measured in lysimeters and sediment were used in combination with water-flux estimates to calculate chemical fluxes. Measurements of δ15N[NO₃] were used to evaluate possible sources of nitrate in the unsaturated zone and ground water.

Tritium data indicate that infiltration of post-1952 precipitation was limited to the upper 2 to 3 m at the two rangeland sites (Figure 1). Water fluxes calculated using tritium mass balances and the depth of pre-bomb/post-bomb interfaces ranged from 4 to 6 mm/yr. Because all of the post-bomb tritium detected at the rangeland sites was located in the root zone, it is unlikely these water fluxes were representative of recharge fluxes.

Figure 1 (Click thumbnail to see larger figure)

Post-bomb tritium was detected throughout most of the unsaturated zone at two irrigated corn fields in Kansas (example shown in Figure 2). The tritium profiles were complicated by the presence of tritium-depleted intervals separating upper and lower zones containing post-bomb tritium. The shape of the tritium profiles and history of water-level declines in the area are consistent with the deeper post-bomb tritium resulting from the declining water table. This interpretation implies early, rapid movement of post-1952 precipitation to the water table. Possible mechanisms for the early arrival of modern recharge at the water table are leakage down the annular space of improperly sealed irrigation wells (wellbore leakage) and spatially variable recharge with most recharge occurring in focused recharge zones such as ditches and depressions. Water fluxes calculated using tritium mass balances and the depth of pre-bomb/post-bomb interfaces ranged from 20 to 54 mm/yr at two fields. Piston flow is the assumed mechanism for water movement in the unsaturated zone. However, bypass or preferential flow could have been important if the deep post-bomb tritium was not from the declining water table. In this instance, water fluxes in the unsaturated zone could have been at least 100 mm/yr if post-bomb tritium first arrived at the water table in 2000.

Figure 2 (Click thumbnail to see larger figure)

Tritium profiles in the unsaturated zone under two irrigated cotton fields in Texas generally were characterized by broad peaks in the upper 20 m of the unsaturated zone that gradually decreased to pre-bomb concentrations with depth (example shown in Figure 2). This gradual decline in tritium concentrations with depth may be indicative of preferential flow processes in the unsaturated zone. Larger tritium concentrations in water from deep irrigation wells compared with water-table wells indicates wellbore leakage also may have occurred in those areas. Water fluxes calculated by attributing the tritium center of mass to the 1963 post-bomb tritium peak ranged from 19 to 34 mm/yr at the two fields. A reliable tritium mass balance could not be made because of the uncertainty in the post-bomb tritium content of irrigation water.

Figure 3 (Click thumbnail to see larger figure)

Water fluxes and concentrations of nitrate (Figure 3) in or near the intervals containing post-bomb tritium were used to calculate chemical fluxes in the unsaturated zone. Nitrate fluxes under two irrigated corn fields ranged from 7 to 176 mmol/m²/yr. These fluxes represented 0.5 to 12 percent of the annual average N application at land surface. Nitrate fluxes under two irrigated cotton fields ranged from 2 to 8 mmol/m²/yr, representing 0.1 to 0.4 percent of the annual average N application at land surface. The larger nitrate fluxes under irrigated corn are consistent with the larger nitrate concentrations at the water table under corn compared to cotton (Figure 3). Nitrate fluxes at the rangeland sites were below detection. The δ15N[NO₃] values for water and sediment at the rangeland and irrigated sites (Table 1) are similar and consistent with either a soil-N source or a fertilizer-N source that has been fractionated isotopically or incorporated into plant organic N and re-released. Landowner records indicate anhydrous ammonia and liquid fertilizer were the predominant forms of N applied at the irrigated fields, whereas no anthropogenic N applications are known to have occurred at the rangeland sites.

Atrazine fluxes were 3 to 31 µg/m²/yr at the corn fields and <1.9 µg/m²/yr at the cotton fields. Atrazine fluxes at the corn fields represented <0.1 percent of the annual average applications at land surface. These fluxes are consistent with the concentrations of atrazine plus deethylatrazine measured at the water table for the corn (0.2 to 0.9 µg/L) and cotton (<0.1 to 0.2 µg/L) fields. Concentrations of atrazine at the rangeland sites were below detection.

The water fluxes reported here do not account for wellbore leakage or preferential flow in the unsaturated zone. Advective chemical transport velocities calculated using these water fluxes and measured volumetric water contents range from 0.17 to 0.45 m/yr in Kansas and 0.13 to 0.28 m/yr in Texas. Given the time available for chemical transport (about 45 yr) and the depths to ground water (about 45 m in 2001), these velocities are too small to account for the occurrence of elevated nitrate and atrazine concentrations in ground water. Thus, results from this study indicate wellbore leakage, focused recharge, and (or) preferential flow in the unsaturated zone are likely the primary processes for transporting irrigation water and agricultural chemicals to the High Plains aquifer.

¹ U.S. Geological Survey, Denver, CO, USA
² U.S. Geological Survey, Menlo Park, CA, USA
³ U.S. Geological Survey, Reston, VA, USA