Effect of urban aquifer exploitation on subsurface temperature and water quality.

Groundwater pumping induces changes in the hydrodynamics of aquifer systems. Rapid urbanization and pumping-induced changes in local groundwater flow can change the natural heat and chemical balances of an aquifer. In the Nagaoka Plain, Japan, groundwater is being extracted at 145 × 10(6) m3 /year from a highly permeable aquifer consisting of coarse-grained sediments of Late Pleistocene to Holocene age. We used land-cover and groundwater analyses to investigate the processes that change subsurface temperature and water quality induced by urbanization and intensive groundwater extraction. Comparison of temperature and water quality measurements in 2009 with measurements made between 1977 and 2000 revealed an increase in subsurface temperature (at 18 m depth) of 0.050 °C/year in the main urban area of the Nagaoka Plain, which is equivalent to the rate of increase of mean air temperature during that period. The effects of surface warming are apparent as a warm zone under the urban area. An area with low saturation index of calcite (-3.0 to -2.0) was centered around urban areas in 2009, whereas in 2000 the index there was higher (-1.5 to -0.5). The decrease in this index in the center of Nagaoka City over the last decade is consistent with continuous dissolution of carbonates induced there by changes in recharge water sources due to groundwater pumping. These findings suggest that urbanization, intensive groundwater extraction, and recharge with chemically modified surface water are responsible for changes in thermal and chemical properties under the urban area of the Nagaoka Plain.