EVALUATION OF THE EFFECT OF A GABBRO-GRANITE TERRAIN ON THE CHEMICAL COMPOSITION OF SURFACE WATERS IN A PRISTINE ENVIRONMENT, CENTRAL MASSACHUSETTS

Quabbin Reservation is an ideal site to study effects of the bedrock chemical weathering and the plant cover on the composition of surface waters in a pristine environment. Dickey Brook, a tributary entirely enclosed within the reservation, is a comparatively small watershed that is underlain in part by Prescott Granitic Gneiss and in part by Prescott Gabbro. A 1 mile long subtributary designated as Dickey Brook 6 is also a contact between the two lithologies (possibly a fault contact) and serves as the area of our study. Water at sampling points that cover the entire main stem of Dickey Brook 6 and all the tertiary branches was measured by a multiprobe for 5 water quality parameters and nitrate concentration in the field. Water samples from the same localities were analyzed by Inductively Coupled Plasma spectroscopy for Na, K, Mg, Ca, Fe, and Mn.

The compositional variation within the brook reflects the underlying lithology, the dwell time between precipitation and discharge by the brook and the varying density of the plant cover in different parts of the drainage area. The "granitic" water pH is more acidic, ranging from 5.3 to 6.3, whereas the "gabbroic" water is roughly neutral. Water draining from the gabbro area contains 1.1ppm Mg, 7.6ppm Ca, and around 2ppm of Na compared to 0.07ppm Mg, 0.07ppm Ca, and around 1ppm Na from the granite area. Calcium and magnesium are most concentrated in the gabbro area headwaters and are progressively more diluted going downstream by mixing with waters coming from the granitic zone. The Fe and K levels do not correlate with the underlying bedrock and appear to be controlled by biogenic activities.

Specific conductivity of waters draining from the gabbroic terrain averages 59.55µS/cm in contrast to waters from area underlain by granitic rocks yieding 24.30µS/cm. Nitrates in Dickey Brook are very low generally in the 0.7-3.71mg/L range and are correlated with the release by decay of organic material.