Use of double-mass curves to determine drawdown in a long-term aquifer test in north-central Volusia County, Florida

A test well withdrew water from the upper Floridan aquifer system at an average of 530 gallons per minute from June 1978 to July 1979. Ground-water levels were monitored in wells open to the upper Floridan (artesian) aquifer system and in wells open to the surficial (water-table) aquifer. Double-mass curves were used to separate drawdown from natural water-level fluctuations. Drawdowns in the Floridan were 4.3 feet at a site 350 feet away from the pumped well, 4.1 feet at a site 1,000 feet away, 1.0 foot at a site 1.0 mile away, and 0.4 foot at a site 5.8 miles away. After an initial drawdown of 0.2 foot at a surficial aquifer well 1,000 feet from the pumped well, there was some recovery, and long-term drawdown averaged less than 0.2 foot. In a five-day aquifer test during a period of no rainfall in 1976, the ratio of surficial aquifer drawdown to Floridan aquifer system drawdown reached equilibrum at 0.05. The results of the long-term test indicate that this ratio is the upper limit of long-term drawdown because of capture of evapotranspiration and runoff during the long-term test. (USGS)