James S. Bollinger and Robert A. Hiergesell
Use of Geographic
Information Systems to Assess Groundwater Monitoring Well Integrity in
a Large Well Network at the Savannah River Site
Abstract
Groundwater samples taken from wells that lack well construction integrity
may produce unreliable data. One of the principal well construction
integrity issues is whether the well filter-pack zone breaches an overlying
or underlying confining unit. Examining well geophysical logs and related
well data to identify wells with screen and filter-pack zones that may
breach a confining unit is a time-intensive process. GIS, therefore,
was utilized as an inexpensive screening tool to determine which wells
at the Savannah River Site's administrative and technical area potentially
penetrate into or breach confining units.
Three dimensional gridded surfaces representing the interfaces of aquifer
and confining units were used in conjunction with well construction data
obtained from a relational database in a GIS application written to locate
well screen and filter-pack zone top and bottom elevations within the vertical
hydrostratigraphy. The application then was used to identify wells
with screen and filter-pack zones that potentially breach a confining unit.
Use of these software tools enabled the efficient identification of 66
monitoring wells that may breach a confining unit within the Savannah River
Site's technical and administrative area. Geophysical logs from these
wells will be examined to determine whether they should be removed from
service.
Introduction and Background
The Savannah River Site (SRS) is a large federal facility, approximately
325 square miles in size, operated by the Department of Energy (DOE) in
South Carolina. The site is located along the Savannah River close
to the city of Augusta, Georgia. The location of SRS is illustrated
in Figure 1. Land for the facility was acquired in 1950 and through
the years various facilities have been operated to support national defense
and space exploration. As a result of these activities, there have
been inadvertent releases of contaminants to the environment. Many
waste materials were disposed of in shallow seepage basins, which was considered
a best management practice during those times. Over time, contaminants
have leached from the disposal facilities and have been released to the
subsurface environment.
In the late 1970's and early 1980's, the awareness of environmental contamination
issues began to grow and an extensive environmental monitoring program
was implemented at SRS. The groundwater monitoring program is a subset
of the environmental monitoring program and is conducted primarily for
the purpose of maintaining compliance with the Resource Conservation and
Recovery Act and other associated South Carolina regulations.
The groundwater monitoring program has grown dramatically in the last 15
years, with the number of monitoring wells now totaling approximately 2000,
of which 570 are located in the A/M Area, which is the technical and administration
area of SRS and the focus of this investigation. In addition to monitoring
wells that are utilized for routine groundwater monitoring, the well network
also includes piezometers, research wells, and other special use wells.
The routinely monitored wells are categorized, for regulatory purposes,
as either Point of Compliance wells, Background wells, or Plume Definition
wells. Each of the different well types have a different sampling
frequency and are analyzed for different suites of contaminants.
The specific suites of analyses are selected to coincide with contaminants
that have been disposed of at nearby waste sites.
Groundwater monitoring wells are installed to allow information to be obtained
in specific aquifers. Many wells are part of a network and are routinely
sampled to provide monitoring of the groundwater chemistry for specific
aquifers. Great care must be taken in the well installation process
to ensure that the screen zone is appropriately located within the subsurface
sediments to allow monitoring of the groundwater in the desired aquifer.
Usually all available information is reviewed prior to installation of
a new well. Typical types of information include geophysical logs
from other previously installed wells located near the new well location,
as well as any descriptive geologic logs from these same wells.
At many new well sites, several wells are installed at different depths.
The deepest well is typically drilled and installed first to provide a
geophysical log that can be used to locate screen elevations for the shallower
wells at the same well cluster. After a borehole is drilled, well
screen and casing is lowered into place such that the screen zone is located
within the aquifer that is to be monitored. A monitoring well screen
is typically 5 feet in length. Connected above the screen, and extending
to the land surface, is solid casing. Well screens and casing are
constructed of polyvinyl chloride. Once the well screen and casing are
situated, gravel material is placed in the annular space between the casing
and the borehole wall. Gravel is filled to an elevation several feet
above the screen. An impermeable clay material is placed immediately
above the gravel, above which grout is used to back fill the annular region
to the land surface. These impermeable annular materials prevent
the vertical migration of groundwater immediately adjacent to the well
casing, thus allowing samples to be collected from the well which reflect
groundwater conditions in the aquifer located adjacent to the screen zone.
For various reasons, the construction of wells such that the screen zones
and filter-pack material are perfectly located is sometimes a difficult
task.
The great majority of wells installed at SRS since the early 1980's have
been constructed such that representative samples can be collected.
However, sample analyses from some wells suggest there may be a well construction
problem. If confirmed, such problems might preclude the ability of
these wells to obtain groundwater samples representative of the aquifer
unit they were intended to monitor. In addition, the understanding
of the hydrogeologic system has evolved over the past 15 years as more
information has become available. The interpretation of the 3-dimensional
configuration of the tops and bottoms of aquifers and confining units has
been periodically updated. New interpretations can require the re-assignment
of a well to an aquifer unit different than the one it was originally thought
to monitor. Monitoring wells with a filter-pack zone which breaches
a confining unit provide geochemical and hydrologic data that is not representative
of the groundwater in a single aquifer unit. Moreover, wells with
filter-pack and screen zones that incise confining units provide a preferential
pathway for contaminants in near surface aquifer units to migrate to deeper
units where production wells are typically located. Identification
and evaluation of such wells is critical to the acquisition of reliable
groundwater monitoring data and protection of deeper aquifer units.
This investigation was undertaken with the primary purpose of optimizing
the groundwater monitoring program within the A/M Area. The objective
was to identify wells which are not providing samples that accurately reflect
the groundwater quality in the aquifer the well was installed to monitor.
The approach utilized to investigate the A/M Area monitoring well network
involved examining historical geochemistry data from individual wells and
also comparing well screen and filter-pack elevations with the elevations
of hydrostratigraphic unit top and bottom elevations to locate wells that
might incise an overlying or underlying confining unit. Specifically,
geochemical screenings were conducted to identify patterns of chemical
analyses at individual wells which might indicate a problem with well construction,
including chronically elevated pH and turbidity levels. Because of
the limited space available, this paper addresses only the analysis to
identify wells with screen and filter-pack zones that might incise a confining
unit.
GIS was used to extract pertinent groundwater quality data from available
ORACLE groundwater databases and to display the extracted information spatially.
This same technology was also used to determine the location of well screen
and filter-pack zones within the A/M Area hydrostratigraphic units and
to identify wells that may breach (extend entirely across) confining units
within this system. This approach saved considerable time and monetary
resources over traditional methods that have been used at SRS in the past
to evaluate monitoring well construction integrity.
Hydrogeologic Model
SRS is located within the Upper Atlantic Coastal Plain Province, which
extends from Mississippi to southeastern North Carolina. Beneath
the Atlantic Coastal Plain Province is a wedge of seaward dipping, unconsolidated
sediments which thicken progressively toward the edge of the continental
shelf. In South Carolina, these sediments are more than 1.2 km thick
near the coastline.
Beneath SRS, the wedge of sediments range in thickness from 180 to 370
meters and consists of interbedded gravel, sand, silt, and clay with some
limestone. These sediments were deposited in near-shore marine environments.
Locally, the sediments can be highly heterogeneous and exhibit a significant
degree of variability in texture both horizontally and vertically.
Layers that are regionally extensive tend to thin in the northwest direction
and thicken in the southeastern, or seaward, direction.
The hydrostratigraphic units that occur beneath the A/M Area at SRS are
depicted in Figure 2. These units, in descending order, are: the
Steed Pond Aquifer, the Crouch Branch Confining Zone, and the Crouch Branch
Aquifer. The Steed Pond Aquifer is divided into upper and lower aquifer
zones by a confining unit that is somewhat discontinuous beneath the A/M
Area. The upper aquifer zone is termed the M-Area Aquifer Zone while
the lower aquifer zone is termed the Lost Lake Aquifer Zone. The
confining zone is termed the Green Clay Confining Zone. Beneath the
Steed Pond Aquifer, the Crouch Branch Confining Zone is also subdivided
by a sandy zone, termed the "Middle Sand" Aquifer Zone. The overlying
and underlying confining zones are termed the "Upper Clay" Confining Zone
and the "Lower Clay" Confining Zone, respectively.
Groundwater moves through this system of layered sediments, proceeding
from areas of recharge to areas of discharge. Recharge to the aquifer system
results from deep infiltration of precipitation. Most of the recharge
at SRS moves through the uppermost aquifer units and discharges locally
along the reaches of perennial streams. Some of the recharge water
moves into the deeper aquifers and eventually is discharged farther away
at the regional discharge locations. Contaminants that leach from surface
disposal basins are carried with the infiltrating water and will eventually
pass through the system of sediments described above. The rate of
groundwater movement is relatively slow and the contaminant plume beneath
the A/M Area has not yet reached local discharge zones.
GIS Analysis
Well construction data for the Savannah River Site's network of approximately
2000 groundwater monitoring wells is housed in an ORACLE relational database.
The first step in the GIS analysis of determining which wells in the SRS
A/M Area might breach or penetrate into a confining unit involved accessing
this well construction database to obtain well construction data for the
570 wells included in the study. The well construction database contains
information such as well use, well geographic coordinates, depth to the
top and bottom of screen and filter-pack zones, and materials descriptions
for each filter-pack zone. This information was accessed using Oracle
Structured Query Language (SQL) embedded in ArcView Avenue scripts so that
the data could be imported directly into ArcView for analysis and display.
The resulting well construction data was subsequently used to generate
an ArcInfo point coverage with well construction data needed for the study
included as attributes.
In order to determine the hydrostratigraphic units where the top and bottom
elevations of each filter-pack zone are located for each monitoring well,
a gridded hydrostratigraphic model was first required. This model
consists of eight surfaces representing the elevations at regularly spaced
x,y coordinates of A/M Area hydrostratigraphic interfaces described in
the section above. These surfaces were contoured using EarthVision
software by Dynamic Graphics, Inc. on a 500 ft. regularly spaced mesh and
were converted to ArcInfo Grid coverages as part of this study. Minimum
tension gridding was used in EarthVision to provide the best fit to the
available hydrostratigraphic picks.
ArcInfo LATTICESPOT was then used to determine the vertical elevation on
all eight surfaces corresponding to the x,y location of each well.
This process may be visualized by placing a vertical line at each well
x,y location such that the line penetrates the surfaces directly underlying
the given well as depicted in Figure 3. LATTICESPOT was used to determine
the vertical elevation where the vertical line intersects each surface
and was used to write these elevations to the ArcInfo coverage containing
the well construction data. A total of eight elevation attributes
were added to the well construction coverage corresponding to the eight
hydrostratigraphic surfaces.
The hydrostratigraphic unit containing the top and bottom of each filter-pack
zone was determined using an ARC Macro Language (AML) routine written to
compare the tops and bottoms of the filter-pack zone elevations with the
corresponding elevations on all eight surfaces at each well x,y coordinate.
For each well, attributes were written to the well construction coverage
indicating the hydrostratigraphic unit name corresponding to the top and
bottom filter-pack elevation.
Well filter-pack zones potentially breaching a confining unit were
identified in the AML by comparing the hydrostratigraphic unit containing
the top and bottom filter-pack location. A potential confining unit
breach was identified if the top and bottom screen location hydrostratigraphic
units were different. As a result of this analysis, 66 wells were
identified which potentially incise a confining unit. The location
of these wells is illustrated on Figure 4 overlaid on top of an aerial
photograph of the A/M Area.
Conclusions and Recommendations
This investigation utilized GIS, a 3-D representation of hydrostratigrphic
unit interfaces, and an ORACLE well construction database to conduct an
automated screening of wells to determine the location of their screens
and filter-packs within the hydrostratigraphic system. As a result
of this screening, 66 wells have been identified which may have been constructed
such that their screens or filter-pack zones extend entirely across a confining
unit adjacent to the aquifer they were installed to monitor. These
66 wells represent approximately 11 percent of the wells that were screened.
Considerable time and monetary resources were saved using GIS as a screening
tool to locate wells within the A/M Area network, which may have filter-pack
zones located across confining units. Geophysical logs from these
wells will be examined to determine whether these wells should be evaluated
for removal from service.
References
Gordon, D.E., Johnson, W.F., Kaback, D.S., Looney, B.B, Nichols, R.L.,
Shedrow, C.B., 1987, Characterization Recommendations for Waste Sites at
the Savannah River Plant, DPST-87-667, E.I. du Pont de Nemours & Co.
Savannah River Laboratory, Aiken, SC.
Authors
James S. Bollinger
Savannah River Technology Center
Westinghouse Savannah River Company
Building 786-6A
Aiken, SC 29808
james02.bollinger@srs.gov
Robert A. Hiergesell
Savannah River Technology Center
Westinghouse Savannah River Company
Building 773-42A
Aiken, SC 29808