POTENTIAL FOR SUBSIDENCE ASSOCIATED WITH GROUNDWATER WITHDRAWAL IN THE DENVER BASIN

Land subsidence is an undesirable side effect of withdrawal of groundwater from aquifer-aquitard systems. Several instances have been reported where subsidence is associated with groundwater withdrawal, with subsidence rates as high as 0.3 m/a. McLaughlin (1955) reported a local rapid decline of more than 48 m in the water level in the Arapahoe Formation in the Denver Basin, and Raynolds (2004) has reported that the potentiometric surface in the Arapahoe aquifer in the Douglas County area of the Denver Basin is falling at about 9 m/a. Galloway et al (1999) report subsidence in the Denver area attributed to aquifer compaction caused by groundwater withdrawal, but to date little attention has been given this problem. Is subsidence small but negligible, or could it be a future problem?

A common feature of sites where subsidence is groundwater induced is that water is pumped from sand and gravel aquifers but the sections include a large percentage of high-compressibility clays. Aquifer drainage causes minor subsidence as these are primarily grain-supported; clay aquitard drainage can cause major slow subsidence because of the high compressibility of clays as they lose water. In the Denver Basin two factors are significant for the magnitude and timing of subsidence. 1) The section is relatively poor in clay, and most of the clay has already compacted to shale. 2) The major aquifers, such as the Arapahoe aquifer, have been confined. but in major population and pumping areas, such as Douglas County, are changing from confined to unconfined conditions (Raynolds, 2004). This transition is causing the first draining of the Arapahoe aquifer creating the conditions likely to cause significant subsidence. Compaction over the next few tens of years is not expected to be large, but differential subsidence could cause some problems. The Arapahoe aquifer dips to the east and regional compaction is likely to start in the west: in southern Arizona, similar regional differential subsidence has resulted in disruptions and reversals in agricultural irrigation systems. Smaller-scale differential subsidence has resulted in surface fissures and faulting in southern Arizona resulting in pipeline, road, and other structural damage, and similar damage could occur in the Denver Basin.