A field investigation of groundwater/surface water interaction in a fractured bedrock environment

To facilitate a better understanding of the exchange processes between a fractured rock aquifer and a bedrock stream, a study of groundwater/surface water interaction between Twenty Mile Creek and the local aquifer in Smithville, Ont. was completed. The points of interaction between the creek and the local aquifer were identified and quantified through the use of air-photo interpretation, detailed stream surveys, electrical conductivity and temperature surveys, isotopic analysis, mixing calculations, and point measurements of hydraulic head and discharge obtained using mini-piezometers, seepage meters and weirs. The results indicate that the interaction between Twenty Mile Creek and the local aquifer is extremely limited with greater than 95% of the groundwater within the study area underflowing the creek completely during baseflow conditions. In addition, it was observed that groundwater discharge in a bedrock stream environment occurs primarily through discrete point sources associated with open fractures, as compared to more diffuse, or continuous seepage zones often observed in a porous media environment. The small quantity of groundwater discharging into Twenty Mile Creek through these features has an electrical conductivity and isotopic signature consistent with shallow groundwater and most likely originates in the upper 3–7 m of bedrock. Although previous investigations have identified potentially open vertical fractures within the study area, it is apparent that the fracture network has extremely poor vertical connections, limiting groundwater exchange in the vicinity of the creek. In addition, the low velocity and discharge rate within the creek does not allow for incising into the bedrock. As a result, horizontal fractures do not play a significant role in the exchange process within the study area.