DECADAL TO DIURNAL VARIATIONS IN WATER QUALITY IN THE ALLUVIAL AQUIFER OF THE MIDDLE ARKANSAS RIVER, KANSAS

The total dissolved solids (TDS) concentration of ground water in the alluvial aquifer next to the Arkansas River downstream of Larned, Kansas varies substantially in both space and time. The shallow alluvial ground water flows across the area from west to east. Dissolved solids in the alluvial aquifer and underlying High Plains aquifer west of the river are partly derived from Cretaceous bedrock. The TDS content of water pumped from these unconsolidated aquifers for irrigation west of the river is increased by evapotranspiration, thereby increasing the salinity of the alluvial ground water from the return flow. The TDS concentration of the alluvial ground water flowing east from the irrigated area into the riparian zone is further concentrated by phreatophyte transpiration. Decadal variations in the TDS content of the alluvial ground water near the river depend not only on climatic conditions (drought versus wet periods that affect the TDS of the eastward flow) but also the volume and source of Arkansas River water that interacts with the ground water in the alluvial aquifer. Flow in the Arkansas River originating from Colorado is saline (sodium-sulfate chemical type). During the last 3 decades, however, there has often been very little or no flow from Colorado that has reached the middle Arkansas River in Kansas. High flows from Colorado large enough to cause appreciable bank storage of saline water near Larned occurred only during 1987 and 1995-2000. Seasonal to weekly changes in the salinity of alluvial ground waters occur primarily as a result of bank storage of very fresh water from substantial stormwater flow events in a tributary, the Pawnee River, which joins the Arkansas River at Larned. The initial influx of this freshwater can cause substantial decreases in dissolved solids in the ground water underlying and adjacent to the river on the time span of hours. Diurnal fluctuations in ground-water salinity can occur along the mixing interface of the freshwater in bank storage and pre-existing ground water in response to small water-table changes caused by phreatophyte consumption. Declines in the water-table level from irrigation pumping and phreatophyte consumption during the last couple of decades have contributed to reduced river flow and have magnified the variations produced by bank storage events.