DEM relied on technical input from the Wellhead Protection Program Advisory Committee in developing the delineation methodology. A mapping approach was required that was scientifically defensible, could be applied consistently across the state, and could be applied with the resources available to DEM. The delineations are based on reasonably available information regarding the hydrogeologic environment and the well characteristics. The WHPAs were delineated using the US Geological Survey quadrangle maps at a scale of 1:24000. WHPA maps are available for review at the DEM Office of Water Resources and on the DEM web page at www.state.ri.us/dem.
The WHPA delineation methodology differs depending if the well is completed in bedrock or stratified drift. The WHPA for a bedrock well is a circle with the radius dependent on the well's pump rate. The WHPA for a stratified drift well is a curve in the stratified drift generated by an analytical model with hydrogeologic/topographic mapping in the upgradient till. To a lesser extent, the methodology varies depending on the type of public well, i.e., community, non-transient non-community or transient non-community (see public well definition at end of this document). The methodologies are described in more detail below.
Bedrock Wells
Groundwater flow direction in bedrock is extremely difficult to predict without a site-specific study. Since the groundwater flow direction for these wells is unknown, the WHPA is a circle (size is determined by the pump rate) in order to be certain to include areas upgradient of the well. The Theis analytical model was used to delineate the WHPA for all bedrock public wells (community, non-transient non-community and transient non-community). The input parameters for the Theis equation were the same for each well, except for the pump rate. If the pump rate was unknown or less than 10 gallons per minute, a 10 gallon per minute pump rate was used resulting in a minimum WHPA with a radius of 1750 feet. The following table provides the calculated radius for selected pump rates:
Wellhead Protection Area Radius for Bedrock Wells
Pumping Rate Radius Pumping Rate Radius (gpm) (ft) (gpm) (ft)
10 1750 23 2411 11 1820 24 2440 12 1872 25 2466 13 1920 30 2568 14 1965 35 2638 15 2008 40 2689 16 2087 45 2729 17 2153 50 2760 18 2211 60 2806 19 2261 70 2875 20 2305 80 2985 21 2344 90 3068 22 2380 100 3133
RIDEM WHPA Delineation Methodology, December 2003 1 The Theis model can be expressed as follows:
s=(114.6Q/T)Wu u=(1.87r^2S/Tt)
Where: s = drawdown; 1 foot Q = well pumping rate T = transmissivity; 374 gpd/ft S = storage coefficient; .01 (dimensionless) t = time; 200 days W(u) = well function (dimensionless) r = distance from pumped well 114.6 and 1.87 are coefficients generated for conversion from metric to english units and conducting some simplifying calculations
Note: The original delineation methodology for bedrock wells with a pump rate greater than 10 gpm also included areas beyond the Theis generated circle where groundwater in the overburden flowed into the circle. This aspect of the methodology was re-evaluated in early 2002, and it was concluded that it was inappropriate to include the area beyond the circle in the WHPA delineation. As a result, all bedrock WHPAs delineated by DEM are in the shape of a circle.
Stratified Drift Wells
The WHPA for community and non-transient non-community stratified drift wells was determined by solving the uniform flow equation (UFE) analytical model for the delineation in the stratified drift and by using hydrogeologic mapping in the upgradient till deposits. Site specific input parameters for each well were used in solving the UFE, which can be expressed as follows:
-Y/X = tan(2piKbi/Q)Y Xl=-Q/2piKbi Yl= +-Q/2Kbi
Where: Q = well pumping rate (ft^3/day) K = hydraulic conductivity (ft/day b = saturated thickness (ft) Note: K x b = Transmissivity i = hydraulic gradient (dimensionless) XL = distance to downgradient null point (ft) YL = distance to lateral boundary limit (ft) X,Y = cartesian coordinates of points along the groundwater divide pi = 3.1416...
RIDEM WHPA Delineation Methodology, December 2003 The UFE was solved with the MWCAP module of the US EPA WHPA model software package. The downgradient null point, Xl, is the location on the UFE curve that represents the distance from which a well is pulling groundwater toward itself that would otherwise flow away from the well. As the UFE curve widens on both sides of the well upgradient, it asymptotically approaches the lateral boundary limits of groundwater flow, which are defined by ±Yl.
Depending upon the well's hydrogeologic setting, the UFE curve was extended upgradient as far as one of the following: a groundwater divide, the till/stratified drift boundary, a ten-year time of travel, or a one-mile cutoff. The WHPA was defined by the UFE curve when the curve was terminated by a groundwater divide, ten-year time of travel, or one-mile cutoff. However, in most cases, the till/stratified drift boundary was used as the upgradient limit of the UFE curve. In these instances, the WHPA also included the upgradient till areas through which groundwater flows to the curve as determined by water table information or topography (most cases topography).
In solving the UFE, only one value for K, b, and i can be used, and these values should be representative of the entire area of stratified drift that is to be captured by the UFE curve. Published US Geological Survey mapped data was used to determine the average transmissivity (saturated thickness times hydraulic conductivity equals transmissivity) and hydraulic gradient. These average values were calculated over an area referred to as a 'clip box' using geographic information system software. The clip box is a mathematically determined estimate of the WHPA in the stratified drift. Where published data was not available, transmissivity was determined from pump tests, lithologic logs, or specific capacity data, and the hydraulic gradient was measured manually.
As new community and non-transient non-community stratified drift wells come into service, an interim WHPA defined by a circle with a 2000 foot radius is assigned until DEM provides a delineation in accordance with the above methodology or the water supplier provides DEM with an acceptable delineation.
Other Wells
There are several instances that do not fit into the two categories above. These situations apply to a small number of wells, most of which serve transient systems. Wells in the following categories have a circular WHPA with a radius of 1750 feet: - The geologic formation supplying water to the well is unknown; - Springs; - Wells completed in till; and - Transient non-community wells completed in stratified drift.
Refined WHPAs
The WHPAs delineated by DEM provide a suitable basis for state and local protection efforts. Ideally, the delineations for the community wells and perhaps some of the larger non-transient non-community wells, will be revised in the future using the same methods described above with more site-specific data or by employing more complex methods. DEM will review the results of the model used in refining the WHPA delineation and determine the area to be designated as the WHPA.
Public Well Definitions:
Community Well - serves year-round residents; at least 15 service connections or at least 25 individuals. Examples include municipal wells and wells serving nursing homes, condominiums, and mobile home parks.
Non-Transient Non-Community Well - regularly serves at least 25 of the same persons (not residents) over 6 months of the year. Examples include wells serving schools and places of employment.
Transient Non-Community Well - does not regularly serve the same persons, but does serve at least 25 people at least 60 days of the year. Examples include wells serving restaurants and motels.
AUTHORITY: These Rules and Regulations are adopted in accordance with Chapter 42-35 pursuant to Chapters 46-12, 46-13.1, 23-18.9, 23-19.1, 42-17.6 and 42-17.1 of the Rhode Island General Laws of 1956, as amended.
Value = Y is a defined Wellhead Protection Area Value = N is part of the extent or universal polygon