METHODOLOGY FOR GENERATING DRINKING WATER USAs

INTRODUCTION

The drinking water resources discussion point paper outlines the criteria for determining the types of drinking water resources which are Unusually Sensitive Areas (USAs). The Pipeline and Hazardous Materials Safety Administration (PHMSA) has begun to identify federal and state government data sources and evaluate the quality and usability of the data to generate drinking water USAs. In this paper, we propose a methodology for implementing the drinking water criteria (outlined in the attached flow chart), considering the availability and quality of data. The numbered paragraphs below refer to the numbers on the flow chart. The reader should refer to the drinking water discussion point paper to review the USA process and technical background for the filtering criteria. The Filter Criteria Table tracks which filter criteria were applied to each state.

It is acknowledged that the proposed approach relies heavily on data that may not be readily available. We have tried to consider secondary sources of data whenever possible. Many states are in the process of generating digital tabular and spatial datasets, but often these datasets are incomplete. Part of the process is to identify missing datasets of greatest need and priority, so that a strategy can be devised for their generation. We believe that drinking water resources are the most complicated dataset because of the three-dimensional nature of aquifers.

FILTER CRITERIA #1

If the public water system is a Transient Noncommunity Water System (TNCWS), the water intakes shall not be designated as USAs.

1. Each state has been contacted to request digital data on drinking water intakes (both surface and ground). They have more current data and more detailed information than SDWIS. The state drinking water database will be sorted on "use" to remove those data which are positively non-public water supplies (e.g., irrigation, feedlot).
   
   
2. The next step is to remove TNCWSs. SDWIS appears to be the more consistent source for determination of if a system is a TNCWS. Therefore, the state drinking water database will be cross-referenced with SDWIS by PWS ID# to remove TNCWSs. As a check to determine if a significant number of non-public water supplies are still included in the database, the remaining number of systems in each state database will be compared with the number of CWSs and NTNCWSs for each state in SDWIS.
   
   
3. The database will then be sorted by "source" that is, surface water or ground water, generating two separate databases "SURFACE" and "GROUND" for drinking water resources.

FILTER CRITERIA #2

For CWS and NTNCWS that obtain their water supply primarily from surface water sources, and do not have an adequate alternative source of water, the water intakes shall be designated as USAs.

4. We have not been able to find any national or state source of information on which this filter criteria could be applied. Thus, it will not likely be used at this time.

FILTER CRITERIA #3

For CWS and NTNCWS that obtain their water primarily from ground water sources, where the source aquifer is identified as a Class I or Class IIa, as identified in Pettyjohn et al. (1991), and do not have an adequate alternative source of water, the well head protection areas for such systems shall be designated as USAs.

Applying this criteria requires knowledge of the source aquifer for the well point and whether the source aquifer is a Class I or Class IIa type. We propose the following approach.

Aquifer Classification

The objective is to determine whether the source aquifer for a ground water public water supply is identified as a Class I or Class IIa aquifer type following the descriptions in Pettyjohn et al. (1991) There is no existing database on which this determination can be made. Explanations for each step in the flow chart are listed below.

5. The aquifers must be mapped at a scale no larger than 1:100,000. There may be both surficial and bedrock aquifers mapped. Both will be used where appropriate. We will use partial data sets if only some aquifers are mapped, particularly where the aquifer maps are in digital format.
   
   
6. If the aquifers are not mapped, geological maps will be used as a substitute for determining the aquifer classifications. We have found more digital sources for geology, compared to aquifers.
   
   
7. In this step, we will identify those aquifers which are likely to be Class I or Class IIa as described by Pettyjohn et al. using either the Aquifer or Geology mapped data. The descriptions of each aquifer/geological formation will be reviewed and compared with the descriptions in Pettyjohn et al. The best source for aquifer descriptions is the Ground Water Atlas of the United States, a 14-volume series recently published by the U.S. Geological Survey. The oldest volume was published in 1991. RPI conducted a similar-type of aquifer classification project in 1986 for the EPA Office of Drinking Water where we classified each aquifer according to itÕs potential for producing water with high levels of natural radionuclides. The aquifer classification scheme we used included most of the factors considered by Pettyjohn et al. Thus, we plan to use our original database on aquifer name, hydrogeological characteristics, etc. and update it with the latest information in the Ground Water Atlas of the United States. The result will be a table listing all of the aquifers in each state which meet the descriptions of Class I or Class IIa of Pettyjohn et al. Where there are digital data on the spatial distribution of aquifers or geology, we will reclassify the aquifers or geology and generate a spatial database on the distribution of Class I and Class IIa aquifers. The maps in Pettyjohn et al. have been scanned and will be used as a first check of the reclassification. Any obvious errors will be identified and corrected or flagged for further evaluation.
   
   Enclosed is an example of this type of analysis for the State of Texas, which had a digital aquifer database which showed aquifer outcrops and subcrops. The major differences between the reclassified aquifer map and the Pettyjohn et. al. map are: 1) where Pettyjohn et al. characterized the Ogallala in northern Texas as Class Ic where we called it Class Ia; and 2) Pettyjohn et al. mapped some surficial clay deposits on the lower coastal plain which were not in the digital aquifer data from the State. The reclassified aquifer database provides very good resolution, compared to the Pettyjohn et. al. maps.
   
   As a second check on the classification system, the lists and plots (where available) of the aquifers which meet the Class I or Class IIa descriptions should be sent to each state for their review. All corrections will be entered, and the final database (AQUIFERS, representing the names and spatial distribution of those aquifers which are Class I or Class IIa) will be used for further analysis.
   
   
8. The GROUND drinking water intakes are wells. If the source aquifer for a well is known, and the name matches that in the AQUIFERS database, then the well will be kept as a potential USA. This screen using tabular data is thought to be more accurate than a geographical screen, since site-specific data are interpreted to identify the aquifer source for each well. However, there will be many cases where an aquifer changes from a Class I to a Class III (an aquifer that is overlain by more than 50 feet of low permeability material). This commonly occurs in the coastal plain province in the downdip direction. Therefore, a geographic-based screen will be needed to identify those wells which are actually located in the surficial or shallow part of the aquifer.
   
   As an example, assume that Texas has a database where the source aquifer for each well is identified. Many wells tap into the Carrizo Aquifer, which is a classic Class Ia aquifer, but only where it outcrops. The Carrizo Aquifer extends for a long distance under the coastal plain, under a ever-thickening wedge of sediments where it becomes a Class III aquifer. Only those wells which meet the requirements of 1) having the Carizzo listed as the source aquifer, and 2) plotting within the areas where the Carizzo outcrops would be carried forward in the analysis. There could be wells in the outcrop area for the Carizzo Aquifer that are screened in a deeper aquifer which is not a Class Ia. Thus, it is important that the AQUIFER coverage include the aquifer name and class, so it can be matched with the source aquifer for the well point.
   
   
9. If the well point database does not include source aquifer in the attributes but does include well depth, we will attempt to predict the source aquifer by comparing well depths with known information on aquifer depth. Other techniques may be attempted to compensate for incomplete data sets. Whenever data are generated by extrapolation, they will be flagged. If possible, all such extrapolated data should be reviewed by state groundwater staff prior to becoming official USAs.
   
   
10. We have not found readily available data on which to apply the criteria for "an adequate alternative source of water" for wells. As for surface water intakes, this criteria may not be applied at this time.
    
    Generating Well Head Protection Area (WHPA) Polygons
    
    WHPAs are used to generate USA polygons around well points. WHPAs designated for different "zones". The discussion point paper did not specify which zone to use, thus we propose to use Zone 2 which is the "management" area which the state or community actively manages sources of contamination around the well head. It typically is defined as no less than the three year time-of-travel distance.
    
    The delineation criteria for WHPAs vary according to aquifer type for each state, and often are given as time-of-transport, rather than arbitrary fixed distances. The actual WHPA is often calculated only when needed, and few states have digital data available (although many states are planning to digitize them). There is no current summary of the criteria, thresholds, etc. for delineating WHPAs for each state. Therefore, we propose the following process for generating WHPAs, which are critical to the generation of USAs for ground water sources.
    
    
11. Where the WHPAs have been generated and digitized as polygons, they will be used and designated as "official" WHPAs (O-WHPA).
    
    
12. Where the State provides digital data for well points with an attribute for each well point that specifies the WHPA as a fixed radius around the well point, we will generate polygons and designate them as O-WHPAs.
    
    
13. Where the State has only a digital well point database, a WHPA will be generated (G-WHPA) for each well point using the default 2,000 feet fixed radius, the same default to be used for states without a WHPP.
    
    

FILTER CRITERIA #4

For CWS and NTNCWS that obtain their water primarily from ground water sources, where the source aquifer is identified as a Class IIb, III, or Class U as identified in Pettyjohn et al. (1991), the public water systems that rely on these aquifers shall not be designated as USAs This criteria will be implemented indirectly. That is, those wells that do not meet criteria #3 will not be considered USAs.

FILTER CRITERIA #5

For CWS and NTNCWS that obtain their water primarily from ground water sources, where the source aquifer is identified as a Class I or Class IIa, as identified in Pettyjohn et al. (1991), and the aquifer is designated as a sole source aquifer, an area twice the well head protection areas shall be designated as USAs.

14. There are digital data for most sole source aquifers, and the remaining sole source aquifers could be easily digitized. Thus, this criteria will be easily applied.

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