Arkansas Department of Health
Center for Local Public Health - Environmental Health Branch - Engineering Section

SOURCE WATER PROTECTION PROGRAM

III. Vulnerability Assessments -- Delineation Methods

The ADH's current "Rules and Regulations Pertaining to Public Water Systems" contain minimum criteria on the location, construction, and protection of public water supply sources. Each public water system must own and effectively control an area around each source. This will be referred to as the "Regulated Area." Our SWAP plan is to expand this "Regulated Area" and perform a Vulnerability Assessment on a larger, Phase I Assessment Area. The Phase I Assessment Area will be the focus in order to meet the statutory deadline. Phase I Assessment data will be utilized to prioritize Phase II Assessments in the future.

Delineations will conform to the methodologies described in this section or as outlined in the approved Arkansas Well Head Protection Plan (see Appendix B for reference). The Phase I Assessment Areas will be delineated on either 1:24,000 USGS topographic maps or 1:62,500 maps that have been digitized from aerial photographs. Impoundments, rivers, springs, and Ground Water Under the Direct Influence (GWUDI) wells will be delineated using a basin approach utilizing 1:24,000 USGS topographic maps. Some wells previously delineated under the WHPP have also utilized the basin approach and will be mapped.

Source waters requiring separate special consideration will be delineated using criteria specific to their situation. There may be many of these special cases found in portions of the State. An example is areas where PSOCs are high in density and / or certain ground water and surface water conditions where basin or aquifer characteristics warrant additional effort.

DELINEATION METHODS

The Arkansas SWAP will incorporate standard delineation methods and additional methods that have been presented to and agreed upon in the process of public meetings, and technical and citizens advisory committees. Well delineation methods are those approved for the Arkansas Well Head Protection Program. Other delineation methods used will utilize a systematic approach specific to each source type. This approach will enable systems to establish protection programs specific to their source, customer needs, and / or other concerns.

DELINEATION METHOD BY SOURCE TYPE

Wells: Four methods, approved under the Arkansas Well Head Protection Program, are used to delineate wells. One or more of the methodologies may be utilized in defining the delineation area. The methodologies are as follows:

Arbitrary Fixed Radius: An area as defined in the State of Arkansas’ Wellhead Protection Plan, generally described as an area within a 0.25-mile radius of the well head. This methodology involves drawing a circle with a specific, but arbitrarily chosen, radius around the well. The length of the radius is not scientifically based, but may be based on very generalized hydrogeologic considerations, or on professional judgement. The method is used in the state only where hydrogeologic data for one of the scientifically based methods defined below are not available or usable. The rationale for the selection of the 0.25-mile radius is outlined in an Addendum to this section.

Volumetric: This method uses a modified formula of the volume of a cylinder to calculate the radius (r) of the WHPA. The ADH assumes a 5 year time of travel.

=

= discharge rate of well

= time of travel to well

= 3.14159

= effective porosity of the aquifer (dimensionless)

= thickness of aquifer zone supplying water to the well

This method is used for the unconsolidated rocks of the Coastal Plain and for alluvial deposits of the Interior Highlands.

Mathematical flow equation: The mathematical flow equation most useful to date is the Theis Non-Equilibrium Equation, which requires a knowledge of aquifer hydraulic parameters including transmissivity and storativity. This equation is commonly used in groundwater flow problems and is discussed in most textbooks on groundwater hydraulics or on the theory of aquifer tests (e.g., see the Theory of Aquifer Tests, U.S. Geological Survey Water Supply Paper 1536-E, 1962 by J.E. Ferris, et. al.). This method is used for the unconsolidated rocks of the Coastal Plain and for the alluvial deposits of the Interior Highlands.

Theis Non-Equilibrium Equation

=

Where:

= well function of & =

= radius of WHPA & =

= 3.14159

= drawdown at boundary of WHPA assumed to be 0.5 ft = discharge of well(s)

= time since pumping started

= Transmissivity ()

= Storativity (dimensionless)

 

Hydrogeologic mapping combined with a hydrogeologic budget: This method consists of two steps. The first involves defining, (by the use of topographic and geologic maps), the boundary of the smallest drainage basin containing the well or well field. The second step involves the determination of a simplified hydrologic budget for the basin. If the selected basin is a self-contained hydrologic unit, precipitation on the basin will equal or balance the sum of water losses, assuming no long-term change of storage. If these quantities do not balance, within a reasonable approximation, the basinal boundary is changed to incorporate a larger basin until a balance is achieved. This method is used for the consolidated-rock terrain of the Interior Highlands.

Impoundments (Lakes, Reservoirs, etc.):

Arbitrary Fixed Buffer: This methodology involves drawing a buffer of a specific distance from the maximum water level of the impoundment or the centerline of its tributaries within a fixed radius around the intake. The fixed radial distance will either be based on time of travel or on professional judgement.

Rivers and Streams:

Arbitrary Fixed Buffer: This methodology involves drawing a buffer of a specific distance from the centerline of the stream or its tributaries within a fixed radius of the intake. The fixed radial distance will either be based on time of travel or on professional judgement.

Springs and GWUDI Wells

Arbitrary Fixed Radius: This methodology involves drawing a circle of a specific radius around the spring / well. The radius is not scientifically based, but may be based on very generalized hydrogeologic considerations, or on professional judgement.

 

Hydrogeologic mapping combined with a hydrogeologic budget: This method consists of two steps. The first involves defining (by the use of topographic and geologic maps) the boundary of the smallest drainage basin containing the well / spring. The second step involves the determination of a simplified hydrologic budget for the basin. If the selected basin is a self-contained hydrologic unit, precipitation on the basin will equal or balance the sum of water losses, assuming no long-term change of storage. If these quantities do not balance, within a reasonable approximation, the basinal boundary is changed to incorporate a larger basin until a balance is achieved. This method is used for the consolidated-rock terrain of the Interior Highlands.

The first component of our SWAP will be the delineation of a Phase I Assessment Area for each public water supply source in the State. The Arkansas Wellhead Protection Program (WHPP) will remain unchanged from the current EPA approved program. The SWAP will utilize the delineation and contaminant inventories developed under the WHPP and will incorporate additional data to provide a complete vulnerability assessment. A vulnerability assessment for all surface water sources in the State will be added.

CONJUNCTIVE DELINEATIONS

Most of eastern Arkansas contains aquifers that are confined or have protective caps. However, the Ozark Highlands in northwestern Arkansas and the Ouachita and Athens Plateaus in western Arkansas are characterized by high intrinsic sensitivity. It is in the Ozark Highlands and the Ouachita and Athens Plateaus that the majority of all the springs and GWUDI wells are located. Our experience suggests that in order to provide an accurate analysis of source susceptibility, conjunctive delineations and assessments are necessary for the GWUDI wells and springs located in these physiographic provinces.

Some streams and impoundments are subject to recharge from ground water. We have selected a buffer around these type of sources that is equal to the Phase I Assessment radius for wells. Therefore, additional conjunctive delineation for streams and impoundments is not warranted. (The rationale for the selection of the 0.25-mile radius is outlined in an Addendum to this section.)

DELINEATION OF ASSESSMENT AREAS

The delineation of wellhead assessment areas will be as follows:

Regulated Area - The current Arkansas "Rules and Regulations Pertaining to Public Water Systems" require that a horizontal distance (measured radially from the wellhead) of not less than 100 feet be maintained between any public water supply well and any possible source of contamination. This is a minimum distance that can be increased where local conditions dictate. The actual size of this area may vary dependent on the timeframe of source development and the edition of the Arkansas "Rules and Regulations Pertaining to Public Water Systems" in effect at that time. Therefore, since the protected area is required by state regulation, activities within this area will continue to be regulated by the ADH.

Phase I Assessment Area - An assessment area will be delineated around each wellhead. The arbitrary fixed radius method of delineation will be used to set the boundary of Phase I Assessment Areas at a radial distance of 0.25 miles around each wellhead that has not previously received a site specific delineation under the WHP Program. Site specific delineations will take precedence over arbitrary fixed radius delineations. Refer to the Arkansas WHP Program, Section III, Delineation of Wellhead Protection Areas for further explanation. (Appendix B).

Surface water assessment areas will be delineated according to the type of source. The source types are:

Impoundments (Lakes, Reservoirs, etc.)

Rivers, streams, etc.

Springs

Ground Water Under Direct Influence (GWUDI) of Surface Water wells

Each type of source water shall also be delineated as follows:

Impoundments (Lakes, Reservoirs, etc.) -

Regulated Area - The current Arkansas "Rules and Regulations Pertaining to Public Water Systems" require public water supply reservoirs to "…own and effectively control a minimum restricted buffer zone including all land bounded by a fixed line which is at least 300 feet horizontally from the shore line when the reservoir is at the maximum high water level contour as established by the ADH". In the case of large multi-purpose reservoirs developed by the federal government, all shoreline within a 0.25-mile radius of the intake must meet the above-described 300-foot buffer zone. The actual size of this area may vary dependent on the timeframe of source development and the edition of the Arkansas "Rules and Regulations Pertaining to Public Water Systems" in effect at that time. Therefore, since the protected area is required by state regulation, activities within this zone will continue to be regulated by the ADH.

Phase I Assessment Area – The entire watershed of the impoundment within the state boundary will be delineated. Within the watershed of the impoundment, the areas defined by the following criteria will constitute the Phase I Assessment Area:

All lands within a 5-mile radius around the intake that are:

Within 0.25 miles of the shoreline at the impoundment’s high water level, and

Within 0.25 miles of either side of the centerline of all tributaries.

In addition, all lands within a 0.5-mile radius of the intake, regardless of watershed boundaries.

Rivers, streams, etc. -

Regulated Area - The current Arkansas "Rules and Regulations Pertaining to Public Water Systems" establish a minimum restricted buffer zone including all land from the river bank to a line 300 feet back, within a 0.25 mile radius of the intake. The actual size of this area may vary dependent on the timeframe of source development and the edition of the Arkansas "Rules and Regulations Pertaining to Public Water Systems" in place at that time. Therefore, since the protected area is required by state regulation, activities within this zone will continue to be regulated by the ADH.

Phase I Assessment Area - The entire watershed of the intake within the state boundary will be delineated. Phase I Assessment Areas will include all lands within 0.25 miles of either side of the centerline of the river / stream and of all its tributaries within a 3 day time of travel limited by a maximum distance up-gradient from the intake of 20 miles (not to exceed state boundaries). Time of travel (TOT) shall be calculated using median flow conditions and a stream slope determined by the slope of the main stem of the river within the assessment area.

Springs -

Regulated Area - The current Arkansas "Rules and Regulations Pertaining to Public Water Systems" require a minimum restricted buffer zone including all property within a 300 foot radius of the spring enclosure. The actual size of this area may vary dependent on the timeframe of source development and the edition of the Arkansas "Rules and Regulations Pertaining to Public Water Systems" in effect at that time. Therefore, since the protected area is required by state regulation, activities within this area will continue to be regulated by the ADH.

Phase I Assessment Area - An assessment area will be delineated around each spring enclosure. The arbitrary fixed radius method of delineation will be used to set the boundary at a radial distance of 0.5 mile around each spring. If the spring is located in the Ozark Highlands in northwestern Arkansas or the Ouachita and Athens Plateaus in western Arkansas and a surface water body intersects the base 0.5-mile radius, conjunctive delineations will performed. In such cases the delineation and assessment area will be increased to include all area within a radius of 3.0 miles of the spring.

(Note: If the spring is the point of origin for a stream, then that stream will not be considered in the conjunctive delineation and assessment. Additionally, delineation and assessment for any waterbody that is at a lower elevation than the spring outlet should not be necessary.)

Recharge basins for springs are not readily available in Arkansas at this time. Some developmental work is underway in northwestern Arkansas to isolate spring recharge basins. If a recharge basin is isolated on any spring prior to completion of the Phase I Assessments, the delineation of the assessment area may be modified to consider the recharge basin on a case by case basis.

GWUDI wells - (i.e. Wells determined to be under the direct influence of surface water.)

Regulated Area - The current Arkansas "Rules and Regulations Pertaining to Public Water Systems" require that a horizontal distance (measured radially from the wellhead) of not less than 100 feet be maintained between any public water supply well and any possible source of contamination. This is a minimum distance that can be increased where local conditions dictate. The actual size of this area may vary dependent on the timeframe of source development and the edition of the Arkansas "Rules and Regulations Pertaining to Public Water Systems" in place at that time. Therefore, since the protected area is required by state regulation, activities within this area will continue to be regulated by the ADH.

Phase I Assessment Area – An assessment area will be delineated around each wellhead. The arbitrary fixed radius method of delineation will be used to set the boundary of the Phase I Assessment Area at a radial distance of 0.5 mile around each wellhead. Refer to Section III, Delineation of Wellhead Protection Areas for further explanation (Appendix B). If the GWUDI well is located in the Ozark Highlands in northwestern Arkansas or the Ouachita and Athens Plateaus in western Arkansas and a surface water body intersects the base 0.5-mile radius, conjunctive delineations will performed. In such cases the delineation and assessment area will be increased to include all area within a radius of 3.0 miles of the wellhead.

ADDENDUM: Rationale For Selection of 1/4 Mile Fixed Radius from WHPP

Based on an analysis of hydrogeologic information for the aquifers of the State, a radius of 1/4 mile around each public water supply well was selected as the generic delineation boundary for the Arkansas Wellhead Protection Plan. This same methodology will be used for the delineation of assessment areas for wells in the SWAP. The rationale for the choice of the 1/4 mile radius is presented in the following paragraphs.

It is a goal of the ADH, in implementing a WHP program, to establish a zone around each well which will generally provide a comfortable degree of protection/warning if contaminant controls and monitoring are implemented within the boundary area. In a groundwater contamination incident sufficient time will be needed to determine the extent of the problem, determine the appropriate actions needed (e.g.; secure new source, install treatment equipment, etc.), secure funding, design and construct the needed facilities. A boundary, which establishes a 5-year time of travel (TOT), is considered the minimum acceptable time frame satisfactory for that purpose.

In determining that the 1/4-mile radius generally provides at least a 5-year TOT around each well the distribution of PWS wells across the various hydrogeologic environments in the state were considered. WHPAs were calculated for selected PWS wells using site specific methods of delineation.

Arkansas is very diverse in terms of geology and hydrology, but can be generally divided into two major regions; 1) Gulf Coastal Plain and 2) Mountains. The population base served by PWSs in the mountainous region, which consists of the Ouachita and Ozark Mountains, generally depends upon surface water as a source of supply rather than groundwater. The Ozark Mountains are generally a Karst area where shallow groundwater is highly vulnerable to contamination. However most community PWSs in this area (which do not depend on surface water sources) depend on deep wells (e.g.; 1,000+ feet) which tap confined aquifers (i.e.; Gunter and Roubidoux).

The great majority of the PWS wells in the State occur in the Gulf Coastal Plain region. A few of the PWS wells withdraw groundwater from alluvial and terrace deposits which are extensive throughout this area, but most are supplied by older, confined aquifers under artesian conditions. One aquifer, the Sparta Sand, supplies more water for PWS wells in the State than all other aquifers combined.

Average groundwater travel times for the 1/4 mile WHPA boundary in the major Gulf Coastal Plain aquifers were calculated on the basis of hydraulic conductivity values provided by the USGS, hydraulic gradients taken from USGS potentiometric maps (selected from areas having high average values), and estimated porosity values. The results are shown in the following table:

The hydraulic conductivity values in this table were derived from pump tests on wells screened in the more productive zones of the aquifers. Hydraulic gradient information is not generally available on aquifers other than the Alluvium and Sparta. Because of the similarities between the confined Gulf Coastal Plains aquifers, a high value was selected for the Sparta and applied to all of the aquifers except the Alluvium for which independent values were available.

The calculated travel times are probably very conservative. Although localized hydraulic gradients may exceed the value shown in the table, the gradients were selected to generally exceed the value expected across the state.

In addition to considering groundwater travel times for the 1/4-mile boundary area, a fixed radius based upon a 5-year TOT was calculated for each community PWS well with current data in Union County for comparative purposes. This county was chosen because all the community PWS wells are completed into the Sparta Sand, which is the major source of drinking water in the State, and because historic data on pumping rates and screened intervals was available from the USGS. The calculated fixed radius defines an area on the ground surface overlying that portion of the aquifer which would contain the volume of water pumped by the well during a five year period, neglecting the effect of the local hydraulic gradient. Of the 47 active community PWS wells in the county, complete data were obtained on 45, resulting in the following summary: (Refer to the table on pages 3-9 and 3-10 for individual results.)

The mean radius found for a 5-year TOT was 736. These figures may be somewhat conservative because a porosity of 0.25 was used in the calculation (rather than the 0.30 used for the earlier flow velocity calculations) and it was assumed that the aquifer was no thicker than the screened interval.

All of the wells that exceeded the 1/4-mile radius belong to the City of El Dorado and have pumping rates that are exceptionally high when compared to most PWS wells in the State. It is probable that only a few major pumping centers, such as El Dorado, Magnolia, and Pine Bluff, would have WHPAs larger than 1/4 mile when calculated in this manner.

As a further check on the appropriateness of using a 1/4-mile radius, analytical models were run on four of the PWS wells in Union County. The model used, MWCAP, is part of the EPA's WHPA Code which was designed for use in delineating WHPAs on a site-specific basis. These computer runs were somewhat generalized in that a county average hydraulic gradient of 0.0022 was used for each of the wells and, for convenience of comparison, the flow direction is toward the east in each case. However, aquifer thickness and the well pumping rate are individually specified.

The generalized WHPAs delineated with MWCAP all fall within the 1/4-mile radius, except for a very small portion of the delineated area around the Smackover well. There was an attempt to choose a wide range of circumstances for these examples. The Smackover well is more representative of the larger WHPAs, while the Faircrest example is probably more typical of most wells in Union County. Because the hydraulic gradients in Union County are locally high due to a county-wide cone of depression from over pumping of the Sparta Sand, WHPAs delineated in this manner would be expected to be smaller over most of the rest of the State.

These comparative analyses indicate that the choice of a 1/4-mile fixed radius, as the WHPA boundary is appropriate for protection of PWS wells in general within the State.

For information on the national level, try EPA's Source Water Protection site