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.
- 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).
- 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.
- 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.
- 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.
- 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.
- 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.
- 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.
- 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.
- 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.
- 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.
- Where the WHPAs have been generated and digitized as
polygons, they will be used and designated as
"official" WHPAs (O-WHPA).
- 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.
- 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.
- 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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