SHALLOW AQUIFER RECHARGE FROM IRRIGATION CANALS NEAR DILLON, MONTANA

Irrigation canals are thought to significantly enhance groundwater recharge; therefore canal seepage was investigated as a mechanism for recharge in an arid intermountain basin in south western Montana. The study area encompasses 81,200 acres near Dillon, Montana. The main economy is irrigated agriculture, with two major canals supplying irrigation needs. One site was instrumented with monitoring wells at each of the two major canals. Weekly samples were taken for specific conductance, and the water isotopes deuterium and ¹⁸O, at 11 monitoring wells and from the canal water. Results illustrate a hydraulic connection between groundwater levels and canal stage, but specific conductance and isotope data indicate that canal seepage only infiltrates as deeply as the shallow groundwater. Seepage loss measurements suggest that the Westside and East Bench Canals lose about 1.2 and 2.1 cfs/mile, respectively.

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.