EXTENSOMETER DATA AS AN ALTERNATIVE MEANS OF DETERMINING AQUIFER SPECIFIC STORAGE, SAN LUIS VALLEY, COLORADO

As part of the Rio Grande Decision Support System (RGDSS), 15 confined-aquifer piezometers, of which 14 were designed to function also as extensometers, were constructed in the San Luis Valley of Colorado. RGDSS is a data centered decision support system being developed by the Colorado Water Conservation Board and the Colorado Division of Water Resources for water resource planning and administration. The San Luis Valley is an intermontane basin of over 6,000 square miles located in south central Colorado. RGDSS is believed to be the first project to attempt direct measurement of sediment compaction in the Valley.

In addition to their primary function as monitoring points for testing and long-term artesian head observations, each piezometer was equipped with an extensometer consisting of a rigid steel pipe anchored at the bottom of the borehole, extending to ground surface. The extensometers were monitored during 72-hour pumping tests of adjacent irrigation wells. The purpose was to provide the opportunity to collect aquifer compaction data during initial testing and in the future, and to determine if compaction in the confined aquifer and aquitard layers could be correlated to drawdown. This was done because the poorly consolidated sediments of the confined aquifer and the confining layers of the San Luis Valley may be susceptible to compaction. The San Luis Valley sediments have geologic characteristics similar to basin-fill sediments in California valleys and other locations where confined aquifer withdrawals have caused compaction-induced land subsidence. Extensometer depths varied from 271' to 1044'. Mean extensometer depth was 617'.

Compaction was monitored to the nearest 0.001" during each test. In most of the tests a positive correlation was observed between drawdown and compaction. Ten of the extensometer data sets were of sufficient quality that they could be evaluated for specific storage (Ss). Preliminary results of the extensometer data, as compared to traditional pumping test techniques, show that the extensometer-derived estimates of Ss are, on average, approximately half the values derived from pumping test analysis. This may be a result of sediments below the extensometer depth contributing water to the tested well due to upward leakage through aquitard layers.