ABOUT THE STUDY

OBJECTIVES

The major objectives of the Bedrock Aquifer Assessment included:

• Identifying geologic and other factors, such as rock type and fracture characteristics, that affect the yields of bedrock wells in New Hampshire.
• Developing predictive tools and data needed by communities for evaluating the ground-water-development potential of bedrock aquifers in their jurisdictions.
• Producing statewide maps that identify the relative potential ground-water yields of bedrock aquifers.
• Describing the ambient quality of ground water in bedrock throughout the State and identifying areas that raise potential bedrock water-quality concerns such as high iron and manganese.

APPROACH

Assessment of potential water supplies and the quality of the water from the bedrock aquifer is complex because of the wide variation, from one location to another, in bedrock fracture, geologic and geochemical characteristics. In addition, there is a lot of uncertainty in our detailed knowledge of the bedrock aquifer system. Therefore, a variety of analyses and techniques were conducted.

Lineament Analysis –

Lineament analysis is based on the identification of aligned topographic features (potential fracture traces) on the land surface using aerial photographs and other remotely sensed imagery. Four types of imagery were used to analyze the State's land surface. These include:

Click to enlarge image

• Landsat imagery (obtained from satellites that orbit the Earth at an altitude of about 570 miles),
• Side-looking radar (obtained from aircraft flown at an altitude of about 33,000 feet),
• High-altitude aerial photography (flown about 40,000 feet above the land surface), and
• Low-altitude photography (flown about 20,000 feet above the land surface).

Observations in the field were also used to supplement fracture-characteristic data in some locations.

Limitations inherent in the lineament analysis include:

• Locations of the lineaments are approximate (+- 80 to 200 feet depending on the source of the imagery).
• Lineaments are most easily observed in areas where the surficial material is thin.
• Lineaments (theoretically) are restricted to steeply dipping fractures or fracture zones that produce straight-line intersections with the earth surface.
• Lineaments were identified visually by scientists using the imagery described above. Other observers may produce somewhat different results.

Click here for a list of the lineament maps and reports

Statistical Well-Yield Analysis –

The USGS statistically compared the yields of wells in New Hampshire's bedrock aquifer to fracture and well-location characteristics. The data set used for the study was compiled by the State of New Hampshire, and included 20,308 accurately located wells with information on yield, depth, and construction.

Multiple regression, using instrumental variables, was the primary method of analysis. This analysis found that not only is there a physical relation between depth and yield, but depth is, in part, a function of desired yield. Drilling is often stopped when a desired yield is reached. This indicates the need for the use of instrumental variables in the regression analysis. This technique was used to essentially estimate the variance of a population of wells that would exist if yield had not been a function of demand.

Bedrock well yields are highly variable, from well to well, and remain difficult to estimate. Model results are presented in terms of the probability of exceeding a given well yield by a given well depth rather than predicted well yields. In this manner, the uncertainty is incorporated into the results.

The results indicated that well yield is generally lower in wells located on steep hill slopes and hilltops, and at a greater distance from surface-water bodies. Yields were found to be greater in wells located in valleys, in sites with large upgradient topographic drainage areas, and within 100 feet of some types of lineaments. Well yields also correlate with 29 mapped geologic units. For example, the Frontenac and Rye Formations have high yields. This analysis provided the statewide map of bedrock yield potential.

Quadrangle-scale investigations were done to determine the degree to which predictive well-yield statistical relations can be improved by local geologic mapping. Additional geologic, fracture, and lineament data were collected for the Pinardville and Windham, N.H. quadrangles, where data from numerous wells (1,682 and 1,504 wells, respectively) were used to test the value of the additional information. The statistical model was applied with and without the additional data. Yield-probability maps were produced for each of the two quadrangles, with and without the added data. These maps clearly demonstrate the advantage of including detailed geologic map units and fracture-correlated lineaments when predicting well yield.

The bedrock well-yield probabilities should be considered a tool in preliminary resource assessment to help understand factors related to high yielding wells. Areas identified as having a high well-yield probability are not guaranteed to be able to supply a high yielding well. Similarly, high yielding wells may be located in certain areas presently identified as having a low well-yield probability. Site-specific investigations, which were beyond the scope of this study, are required to identify new ground-water-supply wells in the bedrock aquifer.

The well-yield probabilities indicate the probability of a well producing at or beyond a predicted yield if a number of randomly placed wells were drilled in that particular yield probability zone on the map. For example, if an area of the map indicates a “25% to 35% probability of obtaining 40 gallons per minute or more from a 400-foot-deep bedrock well”, then this should be interpreted to mean that if ten randomly placed wells (spaced apart from one another) were drilled 400 feet deep in that probability area, about 3 of them would be expected to have yields in excess of 40 gallons per minute.

It must be recognized that a high yielding bedrock well (greater than 40 gpm) can be found in most areas of the state. Hydrogeologists who specialize in ground-water exploration can and do routinely find high yielding wells in most every region of NH when detailed exploration methods such as site-specific geophysical methods and detailed geologic analyses are employed for the specific purpose of exploiting ground water.

Limitations of the Statistical Well-Yield Analysis

There are a number of specific limitations of the well-yield analysis; these include:

•  Detailed geological mapping, 1:24,000-scale or finer, and geophysical surveys can greatly increase the probability of siting a high-yielding production well. However, detailed geological mapping and fracture correlated lineaments were only available and incorporated into the statistical analysis for 2 of the 213 1:24,000-scale quadrangles in the State (Pinardville and Windham).

•  Datasets used in developing the regression are at varying scales. The Statewide bedrock geologic map (Lyons and others, 1997) and the regional digital elevation model (DEM, used to calculate a regional slope and curvature of the land surface) are at 1:250,000-scale. However, much of the data are at a more accurate 1:24,000-scale. These include the data on well locations and predictor data from detailed geologic mapping (the two quadrangles), proximity to water bodies, and topographic setting (1:24,000-scale local slope and curvature).

•  The topographic features are limited to the accuracy and resolution of the 30-meter USGS Digital Elevation Models (DEM). The DEM data for 7.5-minute units correspond to the USGS 1:24,000 and 1:25,000-scale topographic quadrangle map series. One particular limitation is the fact that the DEM source data contained “streaking” artifacts in the data. The streaking is an artifact of the process used to create the DEMs and appear to be swales in the topography that trend due north-south or due east-west. This occurs primarily in mountainous areas. They are artifacts and do not represent actual swales. Since well-yield probabilities can be higher in swales, yield probabilities are likely overestimated where the streaking occurs.

The bedrock well-yield probabilities reflect imperfect datasets and knowledge of the factors related to well-yield in the fractured bedrock aquifer. Model results could be updated as additional data become available and incorporated into possible future analyses. Potential sources of new data include additional geologic and fracture mapping at 1:24,000-scale, more accurate digital elevation models and derivative products, and additional well-yield data.

Geophysical Analysis –

Geophysical methods include borehole and surficial techniques and are useful for identifying fracture zones and the probability of obtaining a high-yielding well. Bedrock-fracture zones near six high-yield bedrock wells in southern New Hampshire well fields were located and characterized using seven surface and six borehole geophysical survey methods.

Detailed surveys of the six sites with various methods provide an opportunity to integrate and compare survey results. Borehole geophysical surveys were conducted at three of the sites to confirm subsurface features. A variety of bedrock and surface hydrogeologic settings were evaluated to gain an insight into the usefulness of the methods in varied terrains.

The surface geophysical methods assess the physical properties of fractured bedrock. Seismic refraction and ground-penetrating radar (GPR) were used mainly to characterize the overburden materials. Magnetometer surveys were used to obtain background information and to search for magnetic lows, which may result from weathered fractured rock. Electromagnetic terrain conductivity surveys (EM) and very-low-frequency electromagnetic surveys (VLF) were used as rapid reconnaissance techniques with the primary purpose of identifying electrical anomalies, indicating potential fracture zones in bedrock. Direct-current (dc) resistivity methods provided the most detailed subsurface information about fracture depth and orientation.

Click here for the report that describes the geophysical analysis

Statistical Water-Quality Analysis –

The USGS statistically analyzed well data relative to bedrock (lithochemical) groupings from 1,353 domestic and 360 public-supply bedrock wells to characterize the variation in ground water quality. The lithogeochemical groupings are mapped bedrock units that have been grouped into lithologic categories on the basis of mineralogical and chemical characteristics relevant to water quality. The domestic-well data used were from homeowner-collected samples analyzed by the New Hampshire Department of Environmental Services (NHDES) Environmental Laboratory from 1984 to 1994.

An understanding of the water-quality conditions of water in bedrock aquifers is important from a public-health perspective because an increasing number of domestic bedrock wells are being drilled and relied upon as a source of drinking water in the State. Bedrock water in New Hampshire often contains high concentrations of iron, manganese, arsenic, and radon gas.

Variations in bedrock water quality were discernable when the data were compared to four major lithochemical groupings of the bedrock, indicating that the type of bedrock has an effect on the quality of water in the bedrock aquifer of New Hampshire. The use of a radon-gas-potential classification of bedrock in the State indicated where high radon concentrations in the air and in water from private and public-supply wells were more likely to occur.

Click here for the report that describes the water-quality data analysis