Measurement of hydraulic head for the evaluation of groundwater recharge to discrete fracture zones in a crystalline bedrock aquifer.

摘要

Groundwater recharge to fractured bedrock has been investigated in only a handful of studies to date. In these studies, it was shown that the estimation of recharge is complex and uncertain. In order to investigate recharge processes in complex bedrock environments, a test site was developed near Perth, Ontario. Three 30 m deep boreholes were completed as multi-level piezometers. The distribution of permeable fracture zones, transmissivities and fracture apertures were estimated from slug testing using straddle packers. Each borehole has between 2 to 3 fracture features whose equivalent single fracture apertures range from (0.28 +/- 0.03) to (1.12 +/- 0.02) mm. In 2005, the discrete fracture zones were monitored for their response to precipitation events. These responses were correlated to real precipitation and barometric pressure data for the local sub-watershed. Hydraulic head spikes, in excess of 3 m, were observed in shallow fracture zones following rainfall events. Any corresponding response within deeper fracture zones was absent in the summer and are slightly delayed during the autumn. Long term barometric efficiencies were determined to be very low, -0.002 to + 0.03, in all monitoring zones, suggesting an unconfined aquifer response. Net recharge was evaluated at two points in the test site using a modified water table fluctuation method, for the autumn monitoring period. The ephemeral recharge signatures that were observed in the shallow zones were effectively ignored. Autumn water levels rose between (1.85 +/- 0.03) and (5.14 +/- 0.03) m. Actual groundwater recharge ranged between (1.78 +/- 0.08) and (2.78 +/- 0.07) mm which corresponded to approximately 1% of the precipitation that fell during the autumn monitoring period. The low recharge estimates correlate to the minimal storage capacity of the aquifer. Further work is required to evaluate the causes of the anomalous water level responses in the shallow fracture zones and the fate of water lost from the recharge process.