Home Services Site Investigation Remedial System Design and Implementation Due Diligence services Geotechnical Engineering Services Environmental Compliance Litigation Support and Expert Testimony Water Supply Services Environmental and Human Risk Characterization Newletter Recruitment About Us Office Locations Links

GeoInsighter Summer/Fall 2001 Newsletter

Municipal Protection of Local Water Resources

Return to the Newsletter Index

Between 1998 and 2020, New Hampshire’s population is projected to grow by more than 342,800 people. Eighty-five percent of this growth is expected to be concentrated on 33 percent of the land area in the state, principally in the southeastern counties. Growth and development is inevitable and essential for many aspects of economic opportunity. But the pace, location, and types of growth that New Hampshire is likely to experience in the next 20 years will have profound and irreversible impacts on the environmental health of the developed land, and of the land’s water resources. While the NHDES protects the quality of public water supplies through well siting regulations (see adjacent article by David Maclean) and requirements to cleanup or mitigate polluted water sources, protection of aquifers through preventative measures is the responsibility of the towns and cities using the aquifers. 

Several municipalities have instituted aquifer protection districts, which restrict the type of land use and density of development over aquifers. The location of the aquifer is often based upon the extent of a permeable soil formation, but in several towns it is also based upon the transmissivity of the soil formation (defined below). The use of transmissivity to define an aquifer protection district may be less protective than the use of the extent of permeable soil formation. 

A municipality’s information on the location and transmissivity of permeable soil formations within town boundaries is typically based upon hydrogeological studies by the United States Geological Survey (USGS), which use surficial geologic mapping of the aquifer materials, transmissivity information and numerical modeling of water withdrawals. Due to the distance between soil borings and wells used in the USGS studies and use of surficial geological information to infer aquifer boundaries between soil borings and wells, further investigation is being contracted by business owners and developers (and by towns in response to business owners’ and developers’ studies) and being brought before town planning boards to alter the boundaries or revise transmissivities as higher-impact property use is pursued. The quality of the studies is an issue that municipalities should be highly sensitive to when making decisions based upon the information presented. The representativeness of a geohydrological study is only as good as the data that it is based upon. 

A geohydrological study should at a minimum include a characterization of the site soil layering, ground water elevations across the property, a description of the testing and calculations performed, the results, recommendations, and example calculations. Aquifer testing may include borehole permeability tests (performed during drilling within the soil boring), slug tests (performed by inserting an inert object within a well and recording water level changes), or pump testing (in which a significant volume of water is removed from the pumped well). Most studies should include, at a minimum, one pump test in which the rate of ground water withdrawal from a well and ground water elevations in the pumping well and other wells (termed observation wells) are recorded.

Pump tests measure an aquifer’s transmissivity, which is the product of the hydraulic conductivity (a measure of a soil’s ability to transmit water) and the thickness of the saturated soil layer. In reality, several soil types and layers are typically saturated, some which transmit water and others that may not. These soils may be interlayered. To accurately represent the transmissivity of a property, soil layering should be known (soil samples should be collected), test wells should be screened throughout the entire aquifer, and the borings for the wells should either confirm the depth to bedrock or identify a sufficient thickness of permeable material to qualify an area as part of the aquifer. The wells are screened throughout the entire aquifer thickness so that the influence of less or more permeable layers will be represented in the data. 

The number and spacing of soil borings and wells installed as part of an investigation should be dependent upon the size of the parcel to be developed, the proposed development usage, and the complexity of hydrogeological conditions. The length of the testing period and number of observation wells is also dependent upon these factors. For existing properties where greater withdrawals are being proposed or the effects on nearby lots are in question, the location of proposed observation wells and the choice of existing wells to use as observation wells is also key to a study’s findings.

Given the growth of the cities and towns in southern New Hampshire, municipal protection of our water resources is becoming a necessity, both to sustain growth and maintain economic opportunity.

Shelley F. Bobowski, P.E.
sfbobowski@geoinc.com

Return to the Newsletter Index