Kansas Geological Survey, Open-file Report 2005-5
D.O. Whittemore, J.J. Butler, Jr., J.M. Healey, S.E. McKay, M.S. Aufman, and R. Brauchler
Kansas Geological Survey, University of Kansas, Lawrence, KS 66047
KGS Open File Report 2005-5
Presented at the 2005 "Water and the Future of Kansas" conference.
Recent studies at a research site in the middle Arkansas River valley downstream of Larned demonstrate the impact of stream-aquifer interactions on ground-water quality. High flows in the Arkansas River infiltrate rapidly into the shallow alluvial aquifer, especially when ground-water levels are low. Thus, changes in the river water quality have a large influence on the quality of the shallow ground water. The salinity of high flows in the Arkansas River downstream of Larned varies appreciably. River flow derived from Colorado is saline, whereas high flows from the Pawnee River are fresh. There has been no significant upstream source of saline water reaching the study site near Larned since late summer 2001, but there have been four short periods during this time (up through fall 2004) when Arkansas River flow at the site has exceeded 100 cubic ft/sec as a result of high flows from the Pawnee River. Conductance measurements in observation wells in the alluvial aquifer reflect the spatial and temporal freshwater influx from these high flows. The freshwater migrates through the upper part of the alluvial aquifer but does not reach the deeper aquifer, as a result of the short-term nature of the high flow and the retardation of downward ground-water flow by clay-rich layers within the aquifer. Heterogeneities within the alluvial aquifer also cause the freshwater influx to move in an uneven spatial pattern.
The dissolved solids concentration in the alluvial ground water before the high-flow events or at a distance from the river channel is substantially greater than in the recent high river flows. The source of the dissolved solids is a mixture of past high flows from Colorado that entered the shallow alluvium, and local ground waters in which the dissolved solids content has increased from evapotranspiration. Ground-water consumption by phreatophytes and irrigation return flow through soils are the major mechanisms responsible for evapotranspiration concentration.
High flows in the Arkansas River (>100 cubic ft/sec) near Larned that are derived from Colorado generally last for longer periods than those caused by Pawnee River storm flow. The longer duration should allow the saline river water to penetrate farther from the river into the shallow alluvial aquifer, as well as deeper into the aquifer. During the last few decades, decreases in Arkansas River flow from Colorado and infiltration of the river water into the alluvial and High Plains aquifers in southwest Kansas have decreased the amplitude and duration of saline high flows reaching Larned. Dry climatic conditions and local and regional irrigation pumping have also contributed to low or no flow conditions in the Arkansas River near Larned. The resulting lower water levels in the alluvial aquifer allow a greater amount of recharge during recent high-flow events, thereby more substantially affecting the alluvial ground-water quality. The substantial spatial and temporal variations in ground-water quality based on observations and inferred from past river-water quality have an important bearing on approaches to ground-water sampling for examination of contaminant transport caused by stream-aquifer interactions.
KGS_Mid_Ark_poster_WFKS-2005.pdf (10.1 MB)
To read this file, you will need the Acrobat PDF Reader, available free from Adobe.