USE OF DEPTH-DEPENDENT SAMPLING TO REFINE UNDERSTANDING OF SOURCE AREAS AND SHORT-CIRCUIT PATHWAYS FOR CONTAMINANTS TO REACH PUBLIC SUPPLY WELLS, HIGH PLAINS AQUIFER, YORK, NEBRASKA 

 

Matthew K. Landon¹ and Brian R. Clark²

 

¹ U.S. Geological Survey, 5735 Kearny Villa Road, San Diego, CA 92123, Tele: 619-778-0135, Email: landon@usgs.gov

² U.S. Geological Survey, 401 Hardin Road, Little Rock, AR 72211, Tele: 501-228-3655, Email: brclark@usgs.gov

 

AGU #10227599, Fall Meeting, December 5-9, 2005, San Francisco, CA.

 

As part of the U.S. Geological Survey’s National Water-Quality Assessment Program, an investigation of the processes controlling the transport of natural and anthropogenic contaminants to public-supply wells has been conducted in layered unconsolidated deposits of the High Plains aquifer near York, Nebraska. Thirty-four monitoring wells with 1.5-m long screens were installed in unconfined and confined layers of the aquifer along or near the zone of contribution to a public-supply well.

 

Analysis of samples collected from the supply well, integrating water from the entire screened interval, indicated concentrations of PCE, TCE, and uranium that were below drinking-water standards but of concern as indicators of contamination. However, these contaminants were not detected in monitoring wells less than 30 m from the supply well in the same confined sand layer. Samples were collected from the supply well under typical pumping conditions at five depths in the 18-m long screen using a submersible 2.2-cm diameter bladder pump lowered down an access pipe. The sample depths were selected based on flow profiling using the tracer pulse method. The samples from the bottom half of the screen had oxygen and hydrogen isotopic values and concentrations of PCE, TCE, major ions, excess nitrogen gas, and uranium consistent with water derived from shallow recharge areas in the urban area mixed with relatively older native water from the confined aquifer. The presence of the unconfined-source water only at the bottom of the supply-well screen implies that well-bore leakage in the supply well itself was not the pathway for contaminant movement. Similar mixed unconfined-source water signatures were detected in a few monitoring wells screened in the confined layer along the zone of contribution. This non-uniform distribution of mixed unconfined-source water implies that there are preferential flow paths that permit urban recharge water and contaminants to move through the confining layer. The primary pathway is probably downward leakage of water through well bores or annular spaces of irrigation or older supply wells that penetrate the confining unit.  

 

The depth-dependent sampling, combined with monitoring-well data, provided improved understanding of the sources and pathways of contaminants and emphasized the need to explicitly simulate well-bore leakage and transient seasonal pumping cycles in a numerical solute-transport model. The results of the simulations indicated that the vulnerability of public-supply wells to contamination is dependent upon preferential pathways and transient seasonal vertical-head gradients.