Historical reconstruction of wastewater and land use impacts to groundwater used for public drinking water on Cape Cod, MA.
This work has recently been published in the Journal of Exposure
Analysis and Environmental Epidemiology.
Introduction | Methods | Data
Sources | Results | Further
Reading | Go
to Town Index |
Source: Swartz, C.H., R.A. Rudel, J.R. Kachajian, J.G.
Brody. (2002). Indicators of wastewater and land
use impacts on public drinking water: Historical reconstruction of exposure
on Cape Cod. Newton, MA: Silent Spring Institute Report.
This webpage documents methodologies
and data developed in the Cape Cod Breast Cancer and Environment
Study to estimate historical wastewater and land use impacts
to drinking water distributed by public water supplies
on Cape Cod, Massachusetts. Data tables and graphs are
available on-line for each town, water supply district,
and well operating during the period 1972 - 1995, and accessed
via a Town
Index map. Data for individual wells is obtained by
clicking on the well location. Precise geographic coordinates
of public water wells are not available on this website.
This work is one component of a multifaceted exposure
assessment for a case-control
epidemiological study of 2100 women that was designed to investigate
the role that exposures to environmental pollutants, including hormonally
active chemicals and mammary carcinogens, may have in explaining elevated
breast cancer incidence on Cape Cod. Previous research has shown that breast
cancer incidence on Cape Cod has been elevated in comparison with other
areas of Massachusetts, even after controlling for established risk factors
(Silent Spring Institute 1997). The Cape Study also incorporates assessment
of historical exposures to wide-area spraying of pesticides on the Cape
for agriculture and insect control (Brody et al. 2002) and personal use
of pesticides (Brody et al. manuscript in preparation) in this investigation
of the role of environmental factors in breast cancer incidence.
Reconstruction of historical water quality for each of the eighteen public
water supply systems serving 150,000 year-round Cape residents is a critical
component of the Cape study, as these systems have historically supplied, and
continue to serve, approximately 80 % of Cape residents. The water distributed
by these systems is drawn almost entirely from a shallow, water table aquifer
underlying the land surface of Cape Cod, and the vulnerability of this aquifer
to contamination from land use activity is well documented (LeBlanc et al.
1986; Janik 1987; Barber et al. 1988; Barlow 1994; Rudel et al. 1998b). Specifically,
on-site sewage disposal in septic tanks and cesspools has been identified as
one of the most serious non-point sources affecting water quality on the Cape
(Belfit 1984), as greater than 90 % of Cape residents dispose of domestic sewage
in cesspools or septic systems. More generally, sewage disposal and its impact
on ground water quality is a critical issue in residential areas throughout
the U.S. (Grady 1993), supporting the need for methods to study potential health
effects of exposure to land use-impacted drinking water and planning tools
that can aid in mitigating future impacts.
The metrics developed in this study to estimate wastewater and other land use
impacts to public water distribution systems incorporate historically measured
chemical parameters for public water supply wells on Cape Cod and analysis
of land use surrounding these wells. The chemical parameters serve as proxy
indicators for the presence of a suite of co-contaminants associated with wastewater
and other land use impacts, a methodology that has been suggested for estimating
historical exposures to disinfection by-products (Arbuckle et al. 2002). In
addition, geographic information system technology was used to estimate impact
to wells through analysis of historical land use within geographic areas delineated
as recharge areas (zones of contribution, or ZOCs) for each of the groundwater
wells supplying the public water distribution systems. Historical land use
within recharge areas to public supply wells was chosen as an indicator of
impact because numerous studies have shown an association between land use
and groundwater contamination and aerial photographs for Cape Cod are available
that indicate land use as early as 1951. These land use-based indicators provide
exposure estimates for time periods prior to the existence of recorded chemical
parameters, and they also supplement information about water quality impact
based on chemical parameters.
METHODS
The 18 public water supply districts on Cape Cod, which are supplied by approximately
145 wells and one surface water source, are included in the analysis. The period
of analysis is bounded by the earliest year (1972) for which measured water
quality parameters were available and the latest year included in the case-control
portion of the Cape Study (1995).
Source Water Quality Characterization
Nitrate concentrations
Nitrate impact for each well was calculated as the annual nitrate concentration
(in mg/L) measured in that well after subtracting 0.2 mg/L, the maximum nitrate
concentration typically observed in unimpacted groundwater on the Cape (Silent
Spring Institute 1997). One study of nitrogen loading to the Cape aquifer found
a median background level of nitrate to be 0.07 mg/L, and water with levels
greater than 0.5 mg/L were characterized as impacted from an anthropogenic
source (DeSimone et al. 1995). Because nitrate is naturally occurring, subtracting
the maximum background concentration from a nitrate measurement produces a
final nitrate level that represents anthropogenic nitrate typically associated
with wastewater and agriculture impacts.
Land use
Land use impacts were assessed by characterizing the amounts and types of land
use that occurred in recharge areas, or zones of contribution (ZOCs), for each
public supply well or group of public supply wells. Land use pertaining to
(1) residential development, (2) routine pesticide applications such as cranberry
bog cultivation, other agricultural applications, golf courses, and railroad
and power line rights of way, and (3) industrial, commercial, waste disposal,
military activities, cemeteries, and transportation features such as major
roads and airports were considered in calculating three different land use
impact values for each ZOC. These impact values were calculated as the percent
land area within each ZOC devoted to each of the three land use groups listed
above. These three fractions are referred to as the ZOC-residential, ZOC-pesticide,
and ZOC-commercial fractions, respectively. These fractions were derived using
standard GIS techniques to overlay ZOC boundaries and land use maps representing
four different time points (1951, 1971, 1984, and 1991) incorporated in the
Silent Spring Institute GIS. To determine ZOC land use fractions for each of
the three land use categories for each of the years falling between the time
points for which land use maps were available, linear interpolation was performed
using the fractions derived for these four time points.
District-wide Water Quality Characterization
Chemistry-based scores
Yearly district-wide nitrate concentrations were obtained by weighting the
annual nitrate concentrations for each well by the ratio of the volume contribution
of that well to the district’s total pumping volume for that year. Note
that the district-NO3 concentrations retain units of concentration after this
weighting procedure. This method of estimating a district impact value from
the nitrate concentrations for wells within each district assumes the water
is homogeneously mixed prior to distribution at the tap. This assumption was
necessary, as information on system hydraulics required to model intra-district
mixing was not available from Cape water suppliers.
Land use-based scores
Unlike the nitrate measurements made at a well, land use surrounding a particular
well in a given year does not reflect concurrent impact to that well, but rather,
represents potential impact to that well at some future point in time. To take
into account the travel time required for land use impacts to affect a neighboring
well, an average time required for contaminants to reach a well from sources
within its zone of contribution was first estimated. This travel time represents
the time between when a contaminant is introduced into an aquifer from a particular
land use and its expected appearance at the neighboring well. This travel time
was then considered in determining which historical year of land use to use
in estimating impact to a well at time points considered for the case-control
study. District-wide impact values for each of the three land use categories
were then calculated by multiplying the particular land use fractional area
for a ZOC for that historical year by the ratio of the cumulative volume contribution
of all wells within that ZOC to the district’s total pumping volume for
the year in which impact was being determined. Again, the assumption of homogeneous
mixing with a district is assumed with this impact estimation method.
The travel time is dependent on three primary variables: (1) the distance between
the well and the contaminant source, (2) the groundwater flow velocity, and
(3) the behavior of the contaminant in the aquifer. The distance between the
contaminant's point of entry into the aquifer and a well is, of course, case
specific. The groundwater flow velocity is dependent on the natural gradient
and any additional gradient that is superimposed on it by the pressure induced
by the pumping well. The behavior of organic contaminants that are the primary
targets in this study depends in large part on their tendency to associate
with, or partition into, natural organic matter distributed among the sediments
composing the aquifer. Significant slowing, or retardation, of organic contaminants
relative to inorganic contaminants such as nitrate can result owing to this
partitioning process.
To derive a travel time for EDCs and mammary carcinogens in the Cape aquifer,
we used nonylphenol as a representative contaminant. Nonylphenol has been documented
as estrogenic and is present in Cape groundwater impacted by wastewater (Barber
et al. 1988; Rudel et al. 1998b). To represent the distance between contaminant
source and a well, we used a constant value equal to half of the average length
(1585 m) of all ZOCs for the Cape. The average distance between a well and
the furthest lateral extent of its ZOC was 3170 ± 1950 m. We used a
retardation factor (Rf) measured for nonylphenol traveling in a secondary sewage
effluent plume on the Cape (Barber et al. 1988) to estimate travel time for
this class of representative compounds. Barber et al. (1988) presents retardation
factors measured for nonylphenol considering both the maximum distance of measurable
concentrations of nonylphenol from the contaminant source (Rf = 2.4) and the
distance to the leading edge of the zone of maximum concentration of nonylphenol
(Rf = 3.3). Assuming a groundwater flow velocity twice (0.6 m day-1) that of
typical natural conditions (0.3 m day-1) (LeBlanc et al. 1986; Barber et al.
1988; DeSimone and Barlow 1995) to account for pumping-induced stress on the
aquifer, it would take approximately 18 to 24 years for nonylphenol to travel
half the average length (1585 m) of ZOCs on the Cape, depending on the retardation
factor used. Note that contaminants such as the volatile organics trichloroethene
(TCE) and tetrachloroethene (PCE) would take approximately 7 to 18 years to
travel this distance based on the range of retardation factors (1.0 and 2.4)
measured in the same study (Barber et al. 1988).
Based upon these calculations, a value of twenty years was used as the travel
time for our target class of compounds across a typical ZOC on the Cape. Thus,
for example, a land use impact value for a well in 1991 would be calculated
using 1971 land use map data within the ZOC for that well; calculation of an
impact value for a well in 1980 would require use of land use data interpolated
for 1960 from the 1951 and 1971 land use maps.
Data Sources
Well Operation Data
Operation Years.
Years in which each of the 132 wells found to be operating on the Cape at some
time during the period studied (1972-1995) were deduced from well operation
data documented by two USGS studies of southeastern Massachusetts water supplies
(LeBlanc et al. 1986; Bratton 1991), well data from Massachusetts Department
of Environmental Protection (MA DEP) (1997) and information provided directly
by water suppliers.
Pumping volumes.
Pumping volumes for wells, used to determine the percent contribution of each
well to its respective district, were obtained from three different sources.
LeBlanc et al. (1986) documents pumping volumes for wells operating on the
Cape for the period 1975-76 and Bratton (1991) provides pumping volumes for
wells operating in 1986. Pumping volumes for wells operating in 1996 were obtained
from MA DEP (1997). Because pumping volume data are readily available for only
the three discrete time points mentioned above, the pumping data for each of
these three time points was assumed to apply throughout the decade in which
the respective data point was documented. These data were augmented by pumping
volume information supplied by district representatives.
Water quality data
Nitrate.
Annual nitrate concentrations were obtained electronically from the Cape
Cod Commission (formerly the Cape Cod Planning and Economic Development
Commission), which has collected these measurements for virtually all public
supply wells on the Cape since 1972. Nitrate concentrations for wells through
1986 are also presented in the State of the Aquifer Report (Janik 1987). Additional
nitrate concentrations were obtained from the MA DEP and directly from water
district representatives. After combining the data from all of these sources,
gaps in nitrate concentrations were still present for some wells, typically
spanning one to several years. For 132 wells over 23 years, there were 17 instances
of a data gap spanning two years, 17 instances of a three to four year gap,
12 instances of a data gap spanning five to nine years, and three instances
of a data gap spanning 10 or more years (14 years was the maximum). Missing
values were interpolated by linear regression of existing data points. Additionally,
a value for the year prior to the first available nitrate measurement (for
years after 1972) was extrapolated by linear regression to reflect the observation
made by district representatives that wells generally began operation the year
prior to the first available nitrate measurement. Nitrate values for each district
were sent to district officials for confirmation.
GIS Coverages
Land use data.
Land use data that identify 26 types of land use including locations of residential,
commercial, industrial, golf course, and agricultural land were developed from
the MacConnell series of land use coverages, which are based on aerial photographs
by the Resource Mapping Project at the University of Massachusetts-Amherst
(MacConnell 1975; MacConnell et al. 1984). Land use coverages were developed
for four years: 1951, 1971, 1984, and 1990. Resolution is three acres for the
1951 coverage and one acre for later years. These four land use maps were also
used in conjunction with town parcel maps and orthophotos to identify the location
and area of airports operating during those time points. Cemetery locations
were obtained from a 1999 land use coverage obtained from MassGIS,
a state office within the Massachusetts Executive Office of Environmental Affairs,
because the maps from earlier time points did not distinguish cemetery locations
uniquely.
Polygon coverages of major roads and highways were created from vector-based
files downloaded from MassGIS. Lines representing roads in these vector-based
files were buffered to the appropriate width (to create polygons) using information
contained in supplementary databases containing road right-of-way width. Road
information was assumed to apply back to the earliest land use coverage time
point (1951).
Power line and railroad rights of way coverages were obtained from MassGIS.
The railroad coverage was augmented with information from historical paper
maps and a coverage obtained from the Cape Cod Commission documenting bike
paths developed on former railroad rights of way. Paper maps with parcel and
street information obtained from public utilities were used to augment the
power line rights-of-way coverage.
Public water supply data.
Datasets locating all public supply wells and ZOCs operating in Massachusetts
were obtained from MassGIS. The ZOCs were approved by MA DEP and defined for
state regulatory purposes as “that area of an aquifer which contributes
water to a well under the most severe pumping and recharge conditions that
can be realistically anticipated (180 days of pumping at safe yield, with no
recharge from precipitation)” (310 CMR 22.02). The datasets comprise
ZOCs, referred to in regulatory context as Zone IIs, delineated through 1996.
These Zone IIs are linked to the public water supply wells used to delineate
the Zone IIs through a unique identification number. A dataset containing additional
ZOCs was obtained from the Cape Cod Commission and combined with the MassGIS
data. A new unique identification number was given to each ZOC in this combined
ZOC dataset. To ensure that all wells were linked to appropriate ZOCs, the
positions of all wells with respect to ZOCs were verified using GIS.
Quality control
Tabulations of annual chemical measurements for individual wells, derived from the Cape Cod Commission and MA DEP databases, were sent to the respective district representatives so that these data could be checked and verified with district records. The values for fractional volume contributions of each well to its respective district, calculated based on literature values as discussed, were also reviewed by district representatives for accuracy.
Well Chemistry
Nitrate
Nitrate concentrations in wells range from values of zero (representing undetectable
levels) to the highest recorded concentration of 6.0 mg/L measured in one public
supply well in 1979. Nitrate levels in wells never surpass the maximum contaminant
level (MCL) of 10 mg/L set by the Environmental Protection Agency in any wells
during the period documented. Nitrate concentrations in a number of wells do
approach 5.0 mg/L, a level set as a regional planning guideline for the Cape
(Cape Cod Commission 1993). Approximately 60 % of the operating wells (81 out
of 132) have had nitrate concentrations greater than or equal to twice the
maximum background level of 0.2 mg/L (i.e., original concentrations of 0.4
mg/L or greater) for two years or more. Approximately 20 % of the source-IDs
(30 out of 132) have had nitrate greater than or equal to ten times the maximum
background level (i.e., original concentrations of 2.0 mg/L or greater) for
two or more years.
Variations in well nitrate concentrations over time can be characterized generally
as (1) having an upward trend with time, (2) being elevated, but with no apparent
trend, and (3) falling near or within maximum background levels (0.2 mg/L)
for the Cape. Trends in nitrate concentration with time are generally distinct.
In contrast, nitrate concentrations in a number of wells remain elevated within
a certain range (typically 1 to 3 mg/L) throughout the study period. Finally,
other wells show little, if any, excess nitrate above background levels.
Statistics (i.e., mean, median, 75th and 90th percentiles) generated for annual
excess nitrate concentrations, considering all wells operating in a given year,
indicate a generally increasing trend in nitrate impact to Cape wells for the
study period. The median excess nitrate concentration rises from 0.00 mg/L
(no impact) in 1972 to 0.31 mg/L in 1995, while the mean concentration increases
from 0.26 mg/L to 0.72 mg/L during this period. Approximately 10% of wells
had excess nitrate concentrations greater than or equal to 1.0 mg/L in 1972,
while more than 25% of wells had 1.0 mg/L or greater nitrate in 1995.
Land Use in ZOCs
While well chemistry data for nitrate indicate impact at the well contemporaneous
with measurement, land use analysis within well ZOCs provides information on
future impact to wells. The fraction of total ZOC area designated for each
of three categories of land use was calculated for this analysis. Residential
land use constitutes the largest fraction of ZOC areas (ZOC-residential fraction)
on Cape Cod for all four time periods analyzed, and the median fraction of
land in ZOCs that is residential rises over the forty year time span from a
value of 2% in 1951 to 23% in 1990. The maximum ZOC-residential fraction observed
over this time period ranges from 33% in 1951 (SSI
ZOC ID 28) up to 80% in 1984 and 1991 (SSI
ZOC ID 50).
The fraction of ZOC land use area on which pesticide application occurs (ZOC-pesticide
fraction) remains relatively constant during the forty year period. Median
values of the ZOC-pesticide fraction range from 5% to 6%. Maximum ZOC-pesticide
fraction values fall also in a narrow range from 24% in 1951 to 21% in 1971
through 1991.
Land use area within ZOCs used for commercial, industrial, waste disposal,
transportation (major roads, highways, and airports), and military purposes
(ZOC-commercial fraction) is generally much smaller than that used for either
residential development or pesticide application, although the mean and median
values for the ZOC-commercial fraction do increase over the time period. Median
values for ZOC-commercial fraction rise from 1.0% in 1951 to 4.0% in 1990,
mean values increase in a similar fashion from 5.0% to 9%. Maximum values for
the ZOC-commercial fraction range from 69% in 1951 to 82% in 1984 and 1990
(all values for SSI
ZOC ID 43, Barnstable Water Company district). The intersection of airport
land use with this ZOC causes the ZOC-commercial fraction for SSI ZOC ID 43
to be so elevated. Several other ZOCs have considerable commercial fractions,
including SSI
ZOC IDs 39 (29% to 58%), 42 (32%
to 58%), and 44 (25%
to 44%) (all in Barnstable Water Company district) and SSI
ZOC IDs 28 (29% to 61%) and 39 (29%
to 58%) in Barnstable Fire district.
District Analysis
District Scores based on Well Chemistry
Concentrations were calculated using the volume fractional contributions of
each well to its respective district for the periods 1972-79, 1980-89, and
1990-95, the annual well nitrate concentrations for the period 1972-95. Note
that district concentrations typically fall when impacted wells were taken
out of operation. The Cotuit district appears to have the highest nitrate concentrations
consistently across the study period, with most values between 1.0 and 2.0.
The Brewster district appears to have the least historical and current impact
from nitrate contamination. District-nitrate concentrations for the Brewster
district are consistently below 0.1.
To facilitate comparison of district-nitrate concentrations among districts
located in proximity to each other, district concentrations are presented in
groups based on geographic location on the Cape. Districts were divided into
the following town groups, in a direction moving from the Upper Cape to the
Lower Cape: (1) the town of Bourne, comprising the North Sagamore, South Sagamore,
Bourne and Buzzards Bay districts, (2) the towns of Sandwich, Falmouth and
Mashpee and their respective districts, (3) the town of Barnstable, comprising
the Barnstable Fire district, Barnstable Water Company district, the Cotuit
district, and the Centerville-Osterville-Marstons Mills district, (4) the towns
of Yarmouth and Dennis and their respective districts, and (5) the towns of
Brewster, Orleans, Harwich, and Chatham, and their respective districts. The
Provincetown district is geographically isolated at the lower Cape from the
other districts.
When the districts are grouped geographically in this manner, some intra-group
similarities can be observed, for the most part for district nitrate concentrations
with time. For the Bourne town group, district nitrate concentrations for both
the North Sagamore and South Sagamore districts are relatively high early in
the 1970s before falling to the range of values (0.0 to 0.5) observed for the
Bourne and Buzzards Bay districts throughout the study period. The Sandwich,
Mashpee and Falmouth districts all have district nitrate concentrations varying
similarly within the range of 0.0 to 0.5. The districts in the town of Barnstable
all appear to exhibit a slight increase in district nitrate concentrations
through the study period, although the ranges over which these increases occur
are variable. Districts for the towns of Yarmouth and Dennis also exhibit an
increase in district nitrate concentrations over the study period, with generally
less inter-annual variability than is observed for the four districts in the
town of Barnstable. The Harwich district, whose wells and zones of contribution
lie near the town boundary with Dennis, exhibits an increase in district nitrate
concentrations very similar in range to that of the Dennis district. Districts
for the towns of Brewster and Orleans both are virtually unimpacted by nitrate
contamination, as indicated by their mutual lack of district nitrate concentrations
greater than 0.1 for the entire study period. The similarity between these
two districts may be due in part to the fact that zones of contribution for
Orleans district wells extend over into the town of Brewster, near the zones
of contribution for Brewster district wells. Nitrate concentrations for the
Chatham district show a smaller increase than those for the neighboring Harwich
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