Depressurization: A Case Study in Faults and Artesian/Water-Table Interactions

Michael R. Thornhill

Depressurizing underlying artesian aquifers is an integral operation in several surface lignite mines in Texas, with some systems pumping 40 million gallons of ground water per day. Failure of a depressurization system could result in catastrophic consequences to mining operations due to floor heave and related conditions.

Complex hydrogeologic conditions in part of one mine posed significant problems in siting successful depressurization wells. Within an area of about one-quarter mile square, artesian conditions, water-table conditions, and faulting exist in the major underlying aquifer. At the downdip portion of the mine block, two intersecting faults form a wedge-shaped area, and geologic and waterlevel data indicate that the aquifer becomes unconfined approximately 1,600 feet updip. Results of aquifer tests showed that negative boundary effects of faulting dramatically increased drawdown in pumping wells within a very short time, significantly decreasing pumping rates. Positive boundary impacts of the water table portion of the aquifer came into effect after about six hours of pumping. These combine boundary effects created a steep gradient, severely limiting the depressurization obtained in the aquifer. In fact, based on the hydraulic characteristics of the aquifer, faulting, limitations in placement of wells, positive boundaries and limited available artesian drawdown, the needed depressurization could not be obtained with conventional well and well field designs.

After considering several more costly alternatives, larger-capacity pumps were set deep within screened intervals of pumping wells to determine if significant increased artesian drawdown could be obtained in the seemingly uniform aquifer in spite of the fact that water-table conditions would be created in the vicinity of pumping wells. Drawdowns achieved and Jacob plots from tests showed that significant additional artesian drawdown was obtained before water-table conditions began to take effect, even though water levels were drawn significantly below the top of the aquifer near pumping wells.

AAPG Search and Discovery Article #91020©1995 AAPG Annual Convention, Houston, Texas, May 5-8, 1995