SHALLOW AQUIFER RECHARGE FROM IRRIGATION CANALS NEAR DILLON, MONTANA
Groundwater levels in regional wells and stream stage were also monitored to compare mountain front recharge with irrigation canal recharge. Both forms of recharge are significant and have independent timing in arid and semi-arid intermountain basins. The local shallow groundwater levels near the canals increased within 5 days after the canals began flowing, while wells influenced by mountain front recharge responded about 2 months later to snowmelt in the upper catchment basin. Tritium results from 16 regional groundwater wells, several of which were located near canals, showed a range of 0.08 to 14.5 TU, suggesting multiple flow systems consisting of local, intermediate, and regional components: (1) local groundwater mixing directly with irrigation recharge; (2) intermediate water recharging since the tritium bomb-pulse; and (3) regional groundwater recharging prior to the bomb-pulse. Although the irrigation canals are located about 3 miles from the Beaverhead River, study results indicated that seepage loss from canals recharges shallow groundwater and augments flow in the river.