LITHOLOGIC CONTROL ON NATURALLY OCCURRING RADIOACTIVITY AND GROUND WATER CHEMISTRY ACROSS THE RICHARDSON MEMORIAL CONTACT, CENTRAL VERMONT

The occurrence of elevated radioactivity in ground water from a public water supply well in Montpelier led to an integrated study of bedrock geology, gamma ray surveys, and ground water chemistry from domestic bedrock wells in the adjacent Montpelier and Barre West 7.5’ quadrangles. The results indicate distinct water quality differences by rock type. The study area straddles the Richardson Memorial Contact (RMC)- an unconformity and major structural boundary that separates Pre-Silurian (PS) rocks of the Green Mountain Anticlinorium from Siluro-Devonian (SD) rocks of the Connecticut Valley Synclinorium. Initial ground-based radiometric surveys indicate that 1) SD units are more radiogenic than PS units and 2) sulfidic phyllites in the SD rocks are more radiogenic than the interbedded impure limestones and micaceous quartzites.

Sixteen of 50 (32%) ground water samples, collected from all major lithologic units in the field area, exceeded the Vermont Dept. of Health (VDH) action level of 5 pci/liter for radioactivity using the gross alpha screening method. Significantly, 94 % (15 of 16) ground water samples that exceeded the VDH action level occur in the SD Northfield, Waits River, and Gile Mountain Formations.

Preliminary analysis of physical and geochemical data for the sampled ground water suggests the following: 1) water in the SD rocks in the Waits River Formation has significantly higher dissolved Ca, K, Mg, Na, and P than water in the PS Moretown Formation; 2) average pH values in the SD formations are higher than in the PS formations because of dissolved CaCO3; 3) average dissolved oxygen values in the PS Moretown Formation are nearly double those in the SD rocks; and 4) average well yields in the Waits River Formation are 30% higher than those in the Moretown Formation.

Ongoing ground water analyses from the 16 wells with elevated gross alpha will quantify amounts of 226Radium, 228Radium, and 238Uranium to determine the source of the elevated radioactivity. Future work will identify which geochemical factors most directly influence ground water radioactivity, with an attempt to develop ground water fingerprints for each of the different formations.