Six to seven percent of the population of Massachusetts obtains its water from domestic bedrock wells. Additional public, commercial, industrial, and domestic supplies from bedrock will be needed in the future. Information about the factors that are related to large well yields is needed. The factors associated with well yields were identified by use of statistical analysis of reported data from 4,218 bedrock wells. The median reported yield of all bedrock wells was 7 gallons per minute, and the median depth was 170 feet. Wells in valleys and lowlands had the largest median yield--I0 gallons per minute. The median well yield on hilltops and slopes was 6 gallons per minute. In valleys and lowlands, significant increases in well yields corresponded to increasing thickness of overburden. On hilltops and slopes, only small increases in well yield corresponded to increases in overburden thickness. Increases in well diameter corresponded to significant increases in well yields for all well locations, depths, and use categories. The common assumptions that fractured crystalline rocks generally yield only small quantities of water to wells and that the fractures that yield water to wells pinch out or are closed because of lithostatic pressure at depths greater than 300 to 400 feet may be in error. Analysis of well data indicates that the median yield of all bedrock wells decreased as well depth increased to 400 feet and increased slightly with well depths greater than 600 feet. The median yield of bedrock wells located in valleys and lowlands reached 50 gallons per minute at depths of 600 to 700 feet. The median yield of wells located on hilltops and slopes reached 15 gallons per minute at depths of 600 to 700 feet. Carbonate bedrock, with a median well yield of 25 gallons per minute, seemed to be the most productive bedrock type. A reported yield of 1,700 gallons per minute from an industrial well completed in carbonate bedrock is the largest reported yield from a bedrock well in Massachusetts. Yield of wells in sedimentary rocks of the Connecticut Valley increased significantly at depths greater than 400 feet, indicating that this bedrock type may have some primary permeability. Commercial or industrial wells had a median yield of 30 gallons per minute. These wells appear to be preferentially sited, large in diameter, and deeper than average to maximize potential well yields. The median reported bedrock well yield and depth have changed over time. The median yield of about 20 gallons per minute for 1920-40 decreased to 6 gallons per minute for 1970-80. Well depth increased from 128 feet for 1950- 60 to 250 feet for 1980-90. The period during which well depths began to increase coincides with a change in bedrock-well drilling methods from cable tool to air rotary with percussion. Four methods of testing the yield of domestic bedrock wells were evaluated: air injection, evacuation and recovery, constant-discharge, and instantaneous discharge or recharge. A constant-discharge test best satisfied the criteria established to evaluate the testing methods. Further data collection and evaluation are needed for the systematic characterization and appraisal of the bedrock aquifers of Massachusetts. Specific elements include (1) improved and expanded collection and storage of well data, (2) detailed hydrogeologic evaluations of apparent high yield bedrock areas, (3) an evaluation of the relation between the vertical distribution of hydraulic properties measured during well drilling to actual hydraulic properties, (4) an evaluation of the effect of well diameter on well yield, and (5) areal evaluations to characterize the quality of water in bedrock.
|Title||Yields of bedrock wells in Massachusetts|
|Authors||B.P. Hansen, A.C. Simcox|
|Publication Subtype||USGS Numbered Series|
|Series Title||Water-Resources Investigations Report|
|Record Source||USGS Publications Warehouse|