Use of isotopic tracers to study pathways for migration of contaminants through the unsaturated zone in irrigated agricultural sites in Southwestern Kansas, USA

Full text: The Great Plains of the United States is a major agricultural region which produces corn and grains. The High Plains aquifer underlies a major portion of the Great Plains and extends from the panhandle of Texas through Nebraska. Since the mid-1950s, irrigation has been utilized in many parts of this region, with water being drawn from the Ogallala Formation. Irrigation has resulted in a draw-down of the water table over most of the system and has introduced agricultural contaminants into the groundwaters, most likely by movement through the unsaturated zone. In the late 1990s, the US Geological Survey NAQWA program began a study of the High Plains aquifer to determine recharge rates and potential contamination problems throughout the system. As part of this program, unsaturated zone studies were carried out at several sites. Included were physical measurements, tritium, stable isotopes of water, and various contaminants in soil waters as well as the concentrations of chlorofluorocarbons in vadose zone gasses. This paper discusses results obtained for unsaturated zone studies at three sites in the central section of the High Plains aquifer in Southwestern Kansas. Two of the sites are agricultural fields (CAL-121 and CAL-122) that grow row crops and have been irrigated with groundwater pumped from the aquifer system since the mid-1950s. Initially, irrigation was carried out by flooding of furrows in the field but the methodology was changed in 1990 to a sprinkler system which is still in use today. Information about crop rotation, chemical usage and the quantity of water used for irrigation is available for both fields. A third location (CNG), about 60 miles southwest of the irrigated sites, was chosen as a control site. This location is a grassland and it has never been irrigated or used for agriculture. Groundwater levels have declined at all sites, although the decline has been greater at the two irrigated sites with depth to water ranging between 45-50 meters at the time of the study. Soil cores were collected using an ODEX drilling method at all sites and sealed samples were returned to the laboratories for extraction and measurement of various parameters. Wells were also installed at all sites to sample the local groundwater. Results from groundwater samples collected at the three sites indicate that recharge to the water table has occurred since the advent of irrigation at the two irrigated locations. Tritium concentrations indicate the presence of bomb-produced tritium in groundwater at both irrigated locations whereas the tritium concentration at in groundwater at CNG was below the detection limit (0.3 Tritium Unit). There were also elevated concentrations of nitrate (up to 24 mg/L as N) and atrazine (0.9 micrograms/L) in the groundwater under the irrigated sites. Concentrations at the control site were 1.2 mg/L and >0.01 micrograms/L respectively for nitrate and atrazine. As there is no possibility of direct recharge for the groundwater, both the tritium and agricultural chemicals must have reached the water table through the unsaturated zone. Tritium analyses on water extracted from core material indicate that water moves through the unsaturated zone at the two irrigated sites much more rapidly than at the control site. Tritium concentrations at the control site dropped from about 7-8 TU to below the detection limit at a depth of about 6 meters. No measurable tritium was found below this depth. Stable isotope results also changed very little below the surface layer at the control site, with 18O averaging about -8.5 per mille throughout the profile. At the irrigated sites, measurable tritium concentrations were found at various depths throughout the core, indicating that post-bomb water was moving through the unsaturated zone. At both irrigated sites, tritium concentrations varied from below the detection limit to over 4 TU at various depths in the profile, indicating that water was not moving down in a simple piston-like fashion. Water may be moving to areas deep within the core via processes such as macropore flow, or by flow around regions where small soil lenses exist where flow is inhibited. Atrazine was occasionally detected in core material at the irrigated sites, usually close to layers where tritium was also present. Calculations using tritium indicate that recharge rates are much greater in the irrigated areas than at the control site. Estimated recharge rates are 54 mm/yr and 39 mm/yr at CAL-121 and CAL-122 respectively, compared to a recharge rate of < 5.4 mm/yr determined at CNG. (author)