THE DENVER BASIN - LEGISLATIVELY SANCTIONED GROUNDWATER MINING

The four principal aquifers within the Denver Basin of eastern Colorado represent a tremendous groundwater resource. This system of multi-layered sandstones and siltstones is estimated to contain over 200 million acre-feet of recoverable groundwater. Encompassing a 6,700 square mile area, the basin underlies portions of 10 counties along the Front Range urban corridor that contain the majority of Colorado's population. Many individuals and municipalities rely on this groundwater for their primary water supply.

The water purity and artesian nature of some of the aquifers was exploited in Denver as early as the 1880s. It would not be until nearly 100 years later, however, that concern over the amount of water being pumped and the life of these deep bedrock aquifers triggered legislative action. In 1985, complex legislation commonly known as Senate Bill 5 required the State Engineer to promulgate rules and regulations governing the withdrawal of groundwater from these aquifers. This legislation re-affirmed a landowner's right to develop deep, non-tributary groundwater underlying their property, adopted a 100-year aquifer life to limit pumping rates, and accepted a known mining condition by not protecting the artesian head or water levels.

Since the passage of Senate Bill 5, most of the counties overlying the Denver Basin have experienced tremendous population growth. Douglas County, for example, has grown from a population of 40,000 in 1985 to over 240,000 in 2005. The number of wells drilled accelerated also with current permitted annual withdrawal rates of 350,000 acre-feet from 4,200 non-exempt and 15,300 exempt wells. In those areas of greatest well density, the pressure heads/water levels in the dominant municipal water supply aquifers, the Arapahoe and Laramie-Fox Hills, are declining. Locally, decline rates in the Arapahoe aquifer exceed 30 feet/year. Declining water levels combined with well-to-well interferences has reduced well productivity by up to 50% during peak summer demands. The development of Denver Basin groundwater was never predicated on the concept of sustainability, rather it was recognized early on that economics would likely be the deciding factor in limiting production of this water resource.