QUANTIFYING SPATIOTEMPORAL GROUNDWATER RECHARGE PATTERNS USING WATER TABLE FLUCTUATIONS AND TEMPERATURE PROFILES

Groundwater depletion is occurring in aquifers worldwide due to unsustainable pumping. Sustainable pumping strategies should be predicated on an understanding of natural groundwater recharge, discharge, and storage processes, but it is difficult to measure groundwater recharge patterns in space and time. Herein, we investigate groundwater recharge patterns through paired ‘heat as a tracer’ and hydraulic approaches via analyses of groundwater level hydrographs and temperature-depth profiles from wells.

Approximately 60% of the population of Cape Breton Island in Nova Scotia, Canada is reliant on groundwater resources abstracted from fractured bedrock aquifers. As Nova Scotia is experiencing increased frequency and intensity of drought as well as pronounced sea-level rise and concomitant risk of saltwater intrusion in coastal aquifers, groundwater management has become increasingly important. One of the major sources of groundwater for the Cape Breton Regional Municipality is a fractured bedrock aquifer that is pumped via the Middle Lake Wellfield. The wellfield (11 production wells) is near the city of Sydney and is currently monitored through an observation well network (17 observation wells) with hourly data logged by transducers. We first calculated recharge using the water table fluctuation method (WTFM) to analyse groundwater elevation measurements (water table and piezometric surface) from wells. These recharge estimates have high uncertainty given the reliance of the method on accurate determination of storage (specific yield or storativity). However, we use the WTFM analysis to investigate patterns in the ratio of recharge to storativity.

To further understand the aquifer dynamics and address uncertainty stemming from the storage coefficient, we recorded temperature-depth profiles in deeper wells using a high-precision RBR SoloT (0.002°C accuracy) temperature logger. The temperature profiles were then analysed using transient and steady-state algorithms based on analytical solutions and applied to constrain recharge estimates. These recharge estimates were then applied to the recharge/storativity ratios from the WTFM analysis to yield storage estimates. This paired hydraulic and thermal approach yields patterns in both storage and recharge rates and provides critical information for sustainable groundwater management.