University of Wisconsin-Stevens Point and University of Wisconsin - Extension
University of Wisconsin-Stevens Point
College of Natural Resources
800 Reserve Street • Stevens Point, WI USA 54481
Investigation of Nitrate in Groundwater Red Springs Area of the Stockbridge-Munsee Reservation
George J. Kraft
David J. Mechenich
Bryant A. Browne
December 10, 2004
The Center for Watershed Science and Education worked cooperatively with the
Stockbridge-Munsee Band of Mohican Indians Environmental Department on an
investigation of groundwater nitrate in the Red Springs Area of the reservation (Figure 1).
The scope of this project involved the following:
1. Reviewing and producing maps of previous sampling data.
2. Producing GIS coverages of nitrate concentrations in relation to land uses and
groundwater flow information.
3. Performing a review of well construction reports for domestic ("homeowner") wells
sampled in previous studies.
4. Producing a database of water quality information from past and current studies.
5. Sampling select points in the monitoring network for nitrate, chloride, and
chlorofluorocarbons (for use in age-dating).
6. Preparing a report.
7. Preparing and delivering up to three educational programs explaining the results of the
study.
This report contains the deliverables reflecting items 1, 3, 5, and 6. GIS coverages (2)
and a database of sampling information (4) are forwarded under separate cover.
Education programs will be delivered upon demand.
Review of Previous Data
The Environmental Department forwarded several Excel spreadsheets to the
Watershed Center that contained construction and water quality data for about 80
sampling points (monitoring wells, tubes, and homeowner wells). The spreadsheets were
merged into a common database and reviewed, revealing that 51 previous sampling
points lay within the Red Springs area. The Environmental Department also forwarded a
an image of groundwater flowpaths as determined by a groundwater flow model (Figure
2) and various GIS coverages.
Several difficulties were noted in the forwarded sampling point data. Five points
had no identification nor location information, five others had no mapping information,
many homeowner wells lacked logs, and the reported amount of standing water in
monitoring wells disagreed among sample dates. We attempted to resolve as many of
these difficulties as possible, and have forwarded more current information to the
Environmental Department. Over the course of the study, one new sampling location
was added so that a universe of 52 sampling points exists in the Red Springs area
(Appendix I). This universe includes 18 homeowner wells, 16 3/8-inch diameter
monitoring tubes, 17 1-inch monitoring wells, and one unknown construction monitoring
1
point. All but four of the 52 points had record of being previously sampled. Previous
samplings revealed that 29% of all sampling points and 47% of homeowner wells
exceeded the drinking water standard of 10 mg/L of nitrate-N (Figure 1).
Description of 2004 Sampling Events
Sampling events were conducted in June and August of 2004. The objective of
the June sampling was to find as many of the previously sample points as possible,
measure the dimensions of monitoring tubes and wells to eliminate database
discrepancies, and collect water samples for analysis of chloride, nitrate, and specific
conductance. Thirty-five points were sampled, comprising 10 1-inch monitoring wells,
12 3/8-inch tubes, and 13 homeowner wells (including one that had not been previously
sampled). Using the data gathered in June, we sampled 10 wells in the August sampling
for CFCs and dissolved oxygen.
For all sampling points, we calculated a "depth below the water table." Depth
below the water table is better than a simple "well depth" for explaining the relationship
of groundwater constituents to depth. Depth below the water table was computed by
subtracting water table depth from the depth of the midpoint of a sample point screen or
open borehole (Appendix I).
2004 Sampling Results
Nitrate and chloride
Nitrate-N and chloride (Table 1) averaged 10 and 12 mg/L over all monitoring
points and 12.8 and 17 mg/L in homeowner wells. The nitrate-N drinking water standard
was exceeded in 37% of all sample points and 47% of homeowner wells. Chloride and
nitrate generally increased together (Figure 3). Nitrate concentrations in individual wells
were about the same in June 2004 as in previous samplings (Figure 4). Nitrate did not
decrease substantially with depth below the water table, though the deepest three
monitoring points (homeowner wells constructed into the granite) had smaller than
average nitrate (Figure 5). Surprisingly, well location with respect to agricultural land
uses did not prove to be a substantial explanatory factor for observed nitrate
concentration. Both large and small nitrate concentrations were observed in areas
mapped agricultural and nonagricultural (Figure 6). Presumably, factors of well depth
and small-scale land use patterns cloud the nitrate-land use relationship.
Dissolved oxygen, denitrification
The ten wells sampled for dissolved oxygen produced values that ranged 0.1 to
9.2 mg/L (Table 1). Four wells had low DOs, less than 1 mg/L. Low DO is significant
because under these conditions groundwater nitrate might be eliminated through
denitrification processes. Denitrification would be evidenced if a low DO well also
contained a large chloride and small nitrate concentration, since nitrate and chloride
correlate. Such evidence was lacking, however, indicating that denitrification was not
2
apparent in the low DO wells. Does evidence of denitrification exist for sampling points
with no DO data? Two sampling points in Figure 3 have small nitrate relative to chloride
and no DO data (N10234 and MW36). Insufficient evidence is available to determine if
the low nitrate - chloride ratio in these is a result of denitrification or just an indicator of a
nitrate-poor chloride source, such as road salt. In summary, evidence is lacking that
denitrification is an effective mechanism for degrading groundwater nitrate in the
sampling points of the Red Springs area.
CFC sampling and groundwater age
The monitoring points for CFC sampling were selected from the pool of
previously sampled points. Selection preferences included an availability of construction
information (many sample points had none) and a greater depth below the water table.
Sampling points with construction information were preferred because groundwater agedate has little meaning without knowledge of what aquifer depth is being represented.
Deeper sampling points were preferred because shallow wells are known a priori to
contain very recent groundwater, so little knowledge is gained by sampling them.
The premise of CFC groundwater age-dating is that the CFC concentration of
groundwater reflects the atmospheric CFC concentration during the year that the
groundwater was recharged. By measuring the CFCs contained in a groundwater sample
and then back-calculating an atmospheric equilibrium concentration, groundwater age
can be estimated by matching the equilibrium concentration with the historical CFC
atmospheric concentration record.
The UW-Stevens Point Dissolved Gas Lab measured concentrations of CFC11,
CFC12, and CFC113 within gas harvested from groundwater. The minimum age-date
estimate of each CFC was bounded by its practical laboratory detection limit. The
practical detection limits were: CFC 11 - 4 pptv (parts per trillion by volume); CFC 12 - 9
pptv; CFC 113 - 1.3 pptv. These correspond to the following minimum age-dates: CFC
11 - 1955; CFC 12 - 1951; CFC 113 – 1959. For CFC 11 and CFC 113, the atmospheric
concentration vs. time curve flattens substantially and then falls between about 1990 and
present. Hence, a given atmospheric-mixing ratio does not correspond to a unique agedate. Therefore, we report an age-date of 1997 for CFC 11 and 113 samples with
atmospheric-mixing ratios corresponding to 1990 to 2004, recognizing an uncertainty of
about + 7 years.
CFC 11, 12, and 113 yield up to three independent observations of apparent age
for each well. However, agreement of all observations is often not attained. In the
Stockbridge-Munsee samples, age dates produced by different CFCs generally agreed
quite well for individual sample points. In two cases, CFC 12 observations were
substantially less than CFC11 and 113 (MW26 and N8311), and in two other cases
interferences from other constituents precluded CFC analyses. CFC113 was not present
above detection limits in N9499 and W10349, probably because groundwater predated
CFC113 concentrations above the practical detection limit. We usually assigned a “best”
age-date for each sample point by averaging the available individual CFC age date
3
estimates. In the case of MW26 and N8311 where CFC12 was anomalously small, the
CFC12 age date was not included in the average.
CFC age dates ranged from 1955 to 1993 (Table 2, Figure 7). Age dates
generally decreased with depth below the water table (Figure 8), but the relationship was
not strong (r2 = 0.30). Wells completed in the granite particularly exhibited substantial
scatter, likely due to the nature of groundwater flow in the granite (probably fracture flow
rather than porous matrix flow), and to the long open borehole in granite wells.
Older groundwater (pre-1964) contained less nitrate than younger (post 1974)
groundwater (Figure 9). This condition is common on agricultural landscapes, and is due
to the large increase in fertilizer nitrogen use that began in about 1960 and did not level
until about 1990.
Conclusion
Sampling in June 2004 revealed that groundwater in the Red Springs area of the
Stockbridge-Munsee reservation is substantially impacted by nitrate pollution. The
nitrate-N drinking water standard was exceeded in 37% of all sampling points and in 47%
of homeowner wells. Nitrate conditions changed little from previous sampling events.
Furthermore, denitrification processes do not appear to be degrading groundwater nitrate.
A key question is how quickly groundwater nitrate will improve in parts of Red
Springs where nitrate sources have mostly been eliminated. The groundwater age-date vs.
depth pattern does not provide a definitive answer. Speculatively, we do not expect that a
substantial improvement will occur over just 5-10 years, but perhaps over 10-20 years, at
least for the drift aquifer. A more sophisticated monitoring network would be needed to
provide a more definitive answer.
Given that nitrate problems will likely be around for some time, what are other
water supply options? One potential option is to utilize nitrate removing treatment
systems, either at the individual homeowner or community system level. Another option
might be to install wells in the granite aquifer. The limited amount of sampling from
granite wells indicated they are generally lower in nitrate than drift wells. This option is
somewhat risky because only a small number (3) of granite wells were sampled, and
because of a danger for encountering radionuclides. Analyses for nitrate and
radionuclides should be performed on any newly drilled granite wells. A third potential
option is to construct relatively shallow wells (but still meeting well construction codes)
immediately downgradient of areas that have not been in nitrate polluting land uses, such
as forest and grassland. This option would need to be monitored closely during and after
well installation to ensure it provides low-nitrate water.
4
Tables
Table 1. Results of nitrate, chloride, specific conductance, and dissolved oxygen analysis.
A negative sign indicates a non-detect. (The detection limit is the absolute value of the
negative number.)
Well no.
MW01A
MW02
MW05
MW07
MW08
MW09
MW12
MW13
MW14
MW15
MW21
MW22
MW24
MW26
MW27
MW29
MW31
MW32
MW36
MW37
MW38
MW39
N8173
N8176
N8311
N8489
N8567
N9499
W10234
W10236
W10239
W10453
W10466
W10475
W10488
Well Type
3/8 inch tubing
3/8 inch tubing
3/8 inch tubing
3/8 inch tubing
3/8 inch tubing
3/8 inch tubing
3/8 inch tubing
1 inch PVC
3/8 inch tubing
3/8 inch tubing
3/8 inch tubing
3/8 inch tubing
3/8 inch tubing
1 inch PVC
1 inch PVC
1 inch PVC
1 inch PVC
1 inch PVC
1 inch PVC
1 inch PVC
1 inch PVC
1 inch PVC
Homeowner
Homeowner
Homeowner
Homeowner
Homeowner
Homeowner
Homeowner
Homeowner
Homeowner
Homeowner
Homeowner
Homeowner
Homeowner
Dissolved
O2
(mg/L)
6.6
8.1
6.4
9.2
2.2
0.7
0.2
Sp. Conductance
(uS)
202
285
332
230
292
342
269
135
280
130
350
496
232
401
69
340
230
330
160
338
350
326
460
241
710
675
870
320
0.1
0.3
385
460
770
648
750
460
5.7
Average all:
Average homeowner:
5
Nitrate-N
(mg/L)
5.6
8.4
3.3
11.2
7.5
6.3
15.6
17.5
-0.1
8.3
17.7
4.1
-0.1
26.9
7.6
26.3
-0.1
1.7
4.3
6.6
5.0
11.4
9.9
7.4
16.0
17.3
39.5
-0.1
0.5
0.3
2.9
20.4
16.1
15.3
8.1
10.0
11.8
Chloride
(mg/L)
1.0
5.5
1.5
6.5
10.5
8.5
11.0
12.0
1.5
3.5
10.5
1.5
2.0
51.5
0.5
14.5
1.5
2.0
23.5
10.0
10.0
10.0
17.5
5.0
19.5
25.0
46.0
0.5
10.5
3.0
21.5
31.0
18.5
16.0
9.0
12.1
17.2
Table 2. Groundwater age date analyses.
Well no.
Well Type
Formation
MW02
MW21
MW26
N8176
N8311
N8489
N9499
W10236
W10239
W10475
3/8 inch tubing
3/8 inch tubing
1 inch PVC
Homeowner
Homeowner
Homeowner
Homeowner
Homeowner
Homeowner
Homeowner
Drift
Drift
Drift
Granite
Drift
Drift
Granite
Granite
Drift
Drift
CFC11
Date
CFC12
Date
CFC113
Date
Best Date
1987
1987
1997
1988
1984
1974
1956
1963
1963
1981
1985
1987
1979
1988
1976
1987
1985
1989
1984
1984
1981
ND
1963
ND
1972
1987
1986
1993
1988
1984
1977
1955
1963
1963
1976
6
1955
1963
1974
#
MW27 U%
#U% MW29
#T$ W10488
#U%
MW30
MW31
#
T$U%
N8567
#U% MW26
U% MW23
U% MW32
T$ #
N8489#
T$ #
#W10453
#U% MW33
U% MW01B
##S MW25
#S
#
S
#
S
#
# MW01A #MW05
MW02
W10466 MW07 N9499
MW06
#S
#T$ W10236
T$
U% N8311
T$ #
S
#
MW03#
#
#
T$
#
S
#
#ST
$
MW24#
MW22
W10475
S
#
MW21#
#U% MW36
MW38U%
T$
#N8176
##T$ N8173
MW37 W10234
T$U%
W10258 #
T$
##Y #U%MW35
#S MW08
#
MW11
#U% MW39
#S
MW09#
##Y MW12
MW13
##S MW10 #U%#T$
W10239
T$ #
#S
N9845
MW14
##S MW16
##S MW15
Well Type
#
Y
#S
U%
ÚÊ
T$
N
0
0
500
1000
0.5
1500 Meters
1 Miles
unknown
3/8 inch tubing
1 inch PVC
Homeowner
MW34
#U%
W9705
#T$
#T$ N8312
Nitrate (mg/l N)
#
#
#
#
#
0.1 - 2
2.1 - 5
5.1 - 10
10.1 - 20
> 20
Agricultural Land Use
Figure 1. Red Springs area and nitrate data from previous samplings. (Average used when more
than one nitrate measurement existed for a sample point.)
7
%
U
$
T
%
U
%
U
%
U
$
T
%
U
%
$
TU
%
U
#
S
#
S
%
U
$
T
#
S
%
$
T U
%
U
#
S
#
S
#
S
$
T
#
S
$
T
$
T
%
U
#
S
$
T
#
S
$
T
%
U
%$
U
T
%
U
$
T
$
T
%
U
$
T
#
S
%
U
#
Y
$
T
#
S
#
Y
%$
U
T
#
S
$
T
$
T
#
S
#
S
#
S
Well Type
# unknown
Y
# 3/8 inch tubing
S
% 1 inch PVC
U
$
T Homeowner
­
0
0
500
1,000
0.5
1,500 Meters
Agricultural Land Use
1 Mile
Figure 2. Red Springs area with USGS model results overlain. Two-foot groundwater
elevation contours and flow paths to monitoring points are shown.
8
Nitrate-N (mg/L)
40
30
y = 0.5743x + 3.0468
R2 = 0.5956
20
10
0
0
10
20
30
40
50
60
Chloride (mg/L)
Figure 3 Relation between nitrate-N and chloride.
June 2004 nitrate-N
40
30
1:1 Trendline
20
10
0
0
10
20
30
40
50
Previous nitrate-N (average)
Figure 4 . Nitrate concentration for points sampled in this study
compared with past samplings. Averages used for past
samplings when more than one existed.
9
40
Nitrate-N (mg/L)
Granite wells
30
20
10
0
0
20
40
60
80
100
120
Depth below water (ft)
Figure 5. Nitrate dependence on depth below the water table.
10
MW27
#U%
#U% MW29
#T$ W10488
#T$U% MW31
N8567
#U% MW26
#T$ N8489 W10453
#T$ #U% MW32
MW01A
S
#
# ##S
##S MW05
MW02
N8311
#
#
#T$
N8176
MW22
S
MW21 #
$
#ST
W10466 MW07
#W10475
MW24 #
ST
$
N9499
#T$ ##S #T$
#T$ W10236
U% MW36
#
U%T
#
#$
N8173 MW37 W10234
#U% #T$
S
#
#MW08
U%
MW39 #
##S MW09
##Y MW12
MW13
#U%#T$
W10239
##S
MW14
MW38
##S MW15
Well Type
#
Y
#S
U%
T$
N
0
0
Figure 6.
500
1000
0.5
1500 Meters
Nitrate (mg/l N)
unknown
3/8 inch tubing
1 inch PVC
Homeowner
#
#
#
#
#
#
No Detect
0.1 - 2
2.1 - 5
5.1 - 10
10.1 - 20
> 20
Agricultural Land Use
1 Miles
Results of nitrate sampling from this study.
11
1993 U%
1977T$
#S
1987
1984
T$
1986 #S
T$1963
1955
T$
T$
1976
T$
1988
T$
1963
Well Type
#
Y
#S
U%
T$
N
0
0
Figure 7.
unknown
3/8 inch tubing
1 inch PVC
Homeowner
XXXX
500
1000
0.5
1500 Meters
Average Age Date
Agricultural Land Use
1 Miles
Groundwater age dates for sampling points.
12
2000
y = -0.2325x + 1986.5
R2 = 0.3013
Age date
1990
Granite wells
1980
1970
1960
1950
0
20
40
60
80
100
120
Depth below water table(feet)
Figure 8. Decrease in groundwater age with depth below the water table.
Nitrate-N (mg/L)
40
30
y = 0.5312x - 1038.9
R2 = 0.5935
Granite wells
20
10
0
1950
1960
1970
1980
1990
2000
Groundwater Age Date
Figure 9. Nitrate N concentrations with groundwater age.
13
APPENDIX I
14
Table A-1. Characteristics of monitoring wells in the Red Springs area. Well
dimensions are those measured in this study.
Well No.
Well Type
MW01A
MW01B
MW02
MW03
MW05
MW06
MW07
MW08
MW09
MW10
MW12
MW13
MW14
MW15
MW16
MW21
MW22
MW23
MW24
MW25
MW26
MW27
MW29
MW30
MW31
MW32
MW33
MW34
MW35
MW36
MW37
MW38
MW39
MW11
3/8 inch tubing
1 inch PVC
3/8 inch tubing
1 inch PVC
3/8 inch tubing
3/8 inch tubing
3/8 inch tubing
3/8 inch tubing
3/8 inch tubing
3/8 inch tubing
3/8 inch tubing
1 inch PVC
3/8 inch tubing
3/8 inch tubing
3/8 inch tubing
3/8 inch tubing
3/8 inch tubing
1 inch PVC
3/8 inch tubing
3/8 inch tubing
1 inch PVC
1 inch PVC
1 inch PVC
1 inch PVC
1 inch PVC
1 inch PVC
1 inch PVC
1 inch PVC
1 inch PVC
1 inch PVC
1 inch PVC
1 inch PVC
1 inch PVC
unknown
Status
Sampled
Not found
Sampled
Sampled
Sampled
Not sampled
Sampled
Sampled
Sampled
Sampled
Sampled
Sampled
Sampled
Sampled
Sampled
Sampled
Sampled
Not sampled
Sampled
Not sampled
Sampled
Sampled
Sampled
Not sampled
Sampled
Sampled
Not sampled
Not sampled
Not sampled
Sampled
Sampled
Sampled
Sampled
Not found
Stickup
(ft)
1.9
Well length
(ft)
Depth to water
(ft)
28.9
2.2
3.4
2.0
30.7
24.2
29.7
13.2
14.9
24.9
17.5
9.3
4.9
1.9
2.2
2.0
2.1
1.4
2.2
2.1
1.3
1.5
2.0
2.0
27.7
29.1
29.2
36.1
30.5
34.7
30.1
25.9
25.6
31.5
29.9
21.9
23.1
20.8
31.8
26.7
28.5
5.8
6.1
8.4
4.4
3.8
6.2
18.6
7.3
11.8
19.5
19.7
10.4
1.6
Standing
water
(ft)
24.3
2.9
2.7
2.8
21.9
13.8
19.1
8.4
9.7
12.0
13.6
4.1
7.1
1.3
2.0
9.3
14.3
4.3
5.6
5.0
8.8
1.5
3.3
3.1
0.7
15.4
28.3
32.7
24.3
7.9
22.9
24.6
16.9
7.5
5.5
8.1
7.4
15
Table A1-2. Characteristics of homeowner wells.
Depth of
well
Diameter
Screen/borehole
(ft)
(in)
length
---- Boucher residence? Poorly documented in database. Not found ---Sampled
Sampled
MH624
125
6
84
Sampled
Not sampled
Sampled
(From
61
6
3
SMED)*
Sampled
Sampled
OB266
105
6
50
Not sampled FO882
122
6
4
Sampled
Sampled
IL572
200
6
155
(From
Sampled
SMED)*
61
6
4
(From
SMED)*
Destroyed
39
6
3
Sampled
Sampled
KQ600
83
6
3
Sampled
MX369
85
6
4
Sampled
Not sampled
Status
None
N8173
N8176
N8311
N8312
N8489
Homeowner
Homeowner
Homeowner
Homeowner
Homeowner
Homeowner
N8567
N9499
N9845
W10234
W10236
Homeowner
Homeowner
Homeowner
Homeowner
Homeowner
W10239
Homeowner
W10258
W10453
W10466
W10475
W10488
W9705
Homeowner
Homeowner
Homeowner
Homeowner
Homeowner
Homeowner
Log number
* Well information provided by Stockbridge-Munsee Environmental Department.
16
Depth to
water
(ft)
Standing water
(ft)
Formation
(ft)
16
109
granite
39
23
sand-clay
18
18
87
104
granite
sand
16
184
28
33
granite
sandgravel
20
19
sand
29
26
54
59
sand
sand