FRACTURE CHARACTERISTICS AND GROUNDWATER ARSENIC IN BEDROCK WELLS OF MANCHESTER, MAINE

High density sampling of domestic wells (1-42 wells per km2) in greater Augusta, Maine, has identified that approximately 40% of wells installed to the calc and calc-silicate meta-sedimentary rock, Sangerville and Waterville Formations, contain > 10 µg/L arsenic. The survey reveals a high degree of spatial heterogeneity of groundwater As distribution at local scales of ≤ 100 m, possibly due to the complexity of groundwater flow patterns and water-rock interaction along fractures. Thin layers (1-10 m) of rock containing hundreds of mg/kg As in sulfide minerals were found in outcrops of the same formations, which could contribute As to domestic boreholes that are typically supplied by water with distinct chemistry from inter-connected fractures. This was investigated by geophysical logging and fracture water sampling in two existing wells installed to the Waterville Formation in Manchester, Maine.

Well MA70076 is 54.3 m deep with 28.3 m casing. Seven fractures, mostly below 45 m, were identified using caliper/conductivity logging and acoustic televiewer logging. Flow meter logging under ambient and pumping conditions showed that 90% of flow to the well was from two dominant fractures at depth of 53.6 m and 51.4 m. The borehole transmissivity was 8-9 ft2/day calculated from water level drawdown and recovery under pumping condition. Water samples were collected from 52 m before the hole was flushed (the hole water) and after (the fracture water) using a dual pumping technique. Fracture water has higher As (24 µg/L with 90% As(V)) and lower Fe (0.2 mg/L) than the hole water with 14 µg/L of As and 0.6 mg/L of Fe.

Well MA70138 is a 58.2 m deep with 32.1 m casing. The well is artesian with a flow rate ~2 gpm. Nine fractures were identified below 45 m, with two dominant fractures at 56 m and 57 m contributing 70% of flow. Water samples were collected from casing (the hole water) and from two fractures at 52 m and 57 m. Fracture waters have higher As (46 µg/L with 90% of As(V) from 52 m and 21 µg/L with 80% of As(V) from 57 m), lower Fe (0.1 mg/L from 52 m and 0.6 mg/L from 57 m) and lower DO (2.2 mg/L from 52 m and 0.7 mg/L from 57 m) than the hole water with 13 µg/L of As, 1.4 mg/L of Fe and 5.2 mg/L of DO.

The lower As concentration of hole water than that of the fracture water is due to loss of As to Fe-particles in the hole because As is found to be enriched in these particles.