BEDROCK GEOLOGY IN NORTHERN KEENE, NH AND HOW IT RELATES TO GROUNDWATER QUALITY

In the vicinity of Keene, NH many household wells are drilled into bedrock. The flow of water in crystalline aquifers primarily occurs between the joints and fractures of the bedrock such that the porosity and permeability can be affected by the presence of brittle faults. In fault zones, the characteristics of the faults and the jointing can differ with each fault and even along the same fault line effecting the flow of water within the aquifer.

In the summer of 2017, an area near the North of Keene was mapped focusing on the identification and characterization of brittle fault structures, silicified fault zones, and jointing. Groundwater and rock samples were also taken in transect across the Mine Ledge Fault (MLF) with the goal of understanding how the bedrock geology effects groundwater flow and quality. Seven water samples from private wells were analyzed for water quality parameters as well as specific major, minor, and trace elements. Eight rock samples were collected from locations near the well sites and analyzed using Inductive Coupled-Mass Spectrometry. The primary element of interest was Iron due to the abundance of Iron Oxides and Hydroxides in the section of the MLF where water samples were taken. The two wells closest to the MLF show elevated concentrations of Iron in comparison to the wells further away. On the contrary, the water and rock sample on the west side of the fault had elevated levels of Manganese while the water sample from the east side did not.

The MLF presents a silicified zone with fault breccia to the North while to the south there is less silicification and more jointing. Even though jointing is present in the Northern section of the fault, locally there seems to be little groundwater flow across the fault. Many of the faults in the study area are normal faults characterized by silicification in the fault plane or zones of bull quartz, however, others show very little silicification and have zones that are heavily fractured and jointed. The structural and mineralogical characteristics of the bedrock play a role in the chemistry of the groundwater. The flow of groundwater within the study area is complexly controlled by the characteristics of the faults such that silicified zones impede the movement of the groundwater.