NMI Site Geology / Hydrogeology

Geology

The geology of the Site includes glacially derived stratified drift deposits underlain by crystalline bedrock.

Site Overburden Geology

The Site topography consists of an irregular series of steeply sided hills with depressions throughout. Some of these depressions, such as the Sphagnum Bog, Cooling Water Recharge Pond and Holding Basin may have been glacial kettles formed by ice-contact sediments deposited around residual blocks of ice. As the blocks melted, the resulting morphology was hummocky. The surface elevation of the Site varies from approximately 137 feet above mean sea level (msl) to 213 feet above msl, rising generally from north to the south.

The predominantly medium sand overburden is thickest (i.e., about 140 to 150 feet thick) at the hill on the northwest corner of the property from GZW-11 to MW-BS28. Where present, the sandy overburden is thinnest (i.e., about 45 feet thick) near ML-3, located in a depression between Route 62 and the northern parking lot for the facility. Below the building foundations, the sandy overburden decreases from about 100 feet thick east of the buildings to about 60 to 80 feet thick west of the buildings. In the region downgradient of the Holding Basin, the sands are fairly homogenous medium sands, with lesser amounts of silt. Sandy overburden below the Holding Basin and Cooling Water Recharge Pond is estimated to be 50 to 60 feet thick.

Till of varying thickness is found below the sandy portion of the overburden at the Site. Up gradient of and beneath the Holding Basin, the till is approximately 5 to 15 feet thick. To the southwest, at MW-BS-10 (adjacent to SW-2A), the till is over 25 feet thick. A till mound north of the former Building D is 20 to 35 feet thick in the area of ML-1 and ML-2.   The till is approximately 30 feet thick at GZW-8 adjacent to the Assabet River.

Site Bedrock Geology

The depth to bedrock has been measured at 30 locations at or near the Site. The bedrock is highest on the eastern side of the Site beneath the Old Landfill and the Sphagnum Bog (110 to 120 feet above msl) and slopes westward to a low of less than 30 feet at MW-BS14 and MW-BS31. There is a bedrock ridge extending from the area north of former Building D to approximately the SW-1 area, from where the bedrock then slopes downward and northward to the Assabet River to an elevation of less than 15 feet at GZW-8-2. From the Holding Basin to the Assabet River, the top of bedrock elevation drops approximately 90 feet. Evaluation of lithology from the pre-RI bedrock borings (seven of eight had core data) indicate the bedrock consists predominantly of gneiss and schist, and, with the exception of the upper few feet of core at ML-1, GZW-6 and GZW-8, the rock is relatively unweathered and unfractured at the locations cored.

Information relative to deeper bedrock structure was collected at three locations: MW-BM03; MW-BM15; and SW-2A. A deep core was collected at MW-BM03 and borehole geophysical logging was completed at MW-BM03 and SW-2A. The RQD values ranged from extremely fractured (0% RQD) to generally massive (100% RQD). At MW-BM03, located near the center of the Site, the rock was highly fractured in the upper 15 ft, and became more competent with depth. At MW-BM15, located near the Assabet River, the rock was more highly fractured with depth. However, the borehole at MW-BM15 was not cored as deep as MW-BM03.

The Bedrock Geologic Map of Massachusetts shows the Site to be underlain by the Assabet Quartz Diorite, with the older Shawsheen Gneiss of the Nashoba Formation to the North and the Andover Granite to the South (Figure 3.1.2). However, evidence from bedrock wells at the W.R. Grace Site north of the Assabet River (GeoTrans, 2002a), the new bedrock map created for a study of the Rockland Avenue Well Site in Maynard (Walsh, 2001), and rock cores from the Site indicate that the Site is most likely underlain by the Shawsheen Gneiss of the Nashoba Formation. Most cores from the Site are described as gneiss, not quartz diorite, and significant sulfide mineralization was observed in bedrock fractures in GZW-8, which is also indicative of the Nashoba Formation. Some core descriptions noted pegmatite zones including GZW-10 and GZW-6. Quartzite or quartz-rich fracture filling was noted in MW-BM03, MW-BS12, MW-BS17 and GZW-8-2. Based on these observations, the State Bedrock Map may incorrectly depict the contact between the Assabet Quartz Diorite and the Shawsheen Gneiss of the Nashoba north of the Site.

Hydrogeology

The Site lies within the Assabet River basin. No natural streams are present on the NMI Property. The only apparent surface water body that pre-dates development of the Site is the Sphagnum Bog located in the eastern-central portion of the Site. The Assabet River flows in an easterly direction and merges with the Sudbury River to form the Concord River approximately 3.5 miles downstream of the Site.

A surface water divide is located in the upland to the south of the Site, surface water runoff from areas north of this divide flow north to the Assabet River. Surface water runoff from areas south of this divide flow south to Second Division Brook, which flows in an easterly direction, and then north to join with the Assabet River.

Groundwater is found both in the unconsolidated and bedrock formations and migrates northward, towards the Assabet River.