1. Nitrate in the Clark Fork Basin (special emphasis on the ground water and surface water in the Summit Valley)
As you may or may not know , the GWA program is currently working in the upper Clark Fork River area. Looking at the preliminary results of our GW sampling the Butte area—Summit valley--stood out in terms of the number of samples that had elevated NO3 concentrations. So with the help of James Rose, Ted and Cam, we did a little digging into the existing data and did a little more sampling to try and determine the extent and the source of the nitrates.
John I. LaFave
Montana Ground-Water Assessment Program
Montana Bureau of Mines and Geology

2. Montana Ground-Water Assessment Characterization Study Areas
Flathead Lake
Scheduled to
Start
Lolo
Bitterroot
Ground-Water Characterization
Ground-Water Characterization study areas are from 1 to 5 counties in size and the work must extend to all parts of each study area. For example, in the Lolo-Bitterroot Area we must collect and interpret basic ground-water data for the Clark Fork River valley below Missoula, the Clark Fork above Missoula, the Potomac area, and the Seeley Lake area as well as the Missoula and Bitterroot valleys.
Ground-Water Characterization provides the basic framework for more detailed ground-water evaluations that may better pertain to site specific problems. However, the Montana Bureau of Mines and Geology attempts to collect some data focused on specific areas of interest in each study area. In the Lolo-Bitterroot study we focused on the Hamilton Heights area to continue work started by the U.S. Geological Survey and to hopefully answer some questions about development on east-side Bitterroot valley benches.
Upper
Clark Fork
Data Collected
Montana Bureau of Mines and Geology
Montana Ground-Water Assessment Program

3. Clark Fork River Basin Geology Reader’s Digest Version
Series of Intermontane Basins.
The basins are structurally down dropped relative to the surrounding mountains.
-Mountains composed of “bedrock”
(fracture porosity.)
-Basins are filled with unconsolidated “basin fill” or “alluvium”
River
Clark Fork River
Flathead
Blackfoot River
The geologic conditions and process have shaped the the landscape and in the broadest sense control the occurrence of ground water in the Clark Fork drainage basin. Those processes include: faulting, folding, volcanism, glaciation, weathering and erosion.
The result being a series of intermontane basins…
Bitterroot River
MBMG – Ground-Water Assessment Program

4. Clark Fork Basin -Wells 1975
Ground water a critical resource: supplies most municipalities and residences
MBMG – Ground-Water Assessment Program

5. Clark Fork Basin -Wells 2005
Ground water a critical resource: supplies most municipalities and residences
~ 50% of wells drilled
In the last 15 years
MBMG – Ground-Water Assessment Program

6. Nitrate in ground water -what are the sources?
Natural
N2 conversion to NO3
Geologic (rare)
Anthropogenic
Non-point sources
Fertilizers
Point sources
Septic systems
There are natural sources of nitrate- N2 fixation (conversion of N2 to NO3) is part of a naturally functioning ecosystem and NO3 can be derived from geologic deposits (shales)- but they typically don’t result in elevated concentrations or are relative rare as in the case of geologic N.
The two main sources of concern with respect to NO3 contamination of gw are overuse of fert. And effluent from septic systems.
Fertilizers can be a concern not only in ag areas but in urban or residential areas as well where they are applied to lawns.
Use of N-based fertilizers has really taken off in the past 50 years – and represents a significant source of N into the environment. It is also significant in that NO3 from fertilizers detected in gw can be a marker of recent water much like tritium.
The other culprit is septic systems, more and more of a concern in intermontane basins of w MT where development along the basin margins—which also represent ground water recharge areas-- is accelerating. Where gw contamination occurs it is typically related to the density of septic systems.
What is background? Results from 10’s of thousands of samples nation wide as part of the USG NAQWA program suggest that 2 –3 mg/L in gw represents the upper threshold of background in gw , and about 0.6 mg/L in surface water.
Background in “natural” systems
< mg/L in ground water
< 0.6 mg/L in surface water
MBMG – Ground-Water Assessment Program

7. Not Including Summit Valley
Distribution of Nitrate
– Clark Fork Basin
Not Including Summit Valley
(710 samples)
2 – 10
mg/L
non detect.
NO3 in most
ground-water
(88% of samples)
less than 2 mg/L
< 2 mg/L
Summit Valley
(47 samples)
NO3 in most
ground-water
(59% of samples)
greater than
2 mg/L
MBMG – Ground-Water Assessment Program

8. Summit Valley N Valley fill - several hundred feet of unconsolidated
alluvium
Ground water flows away
From the valley margins
towards the streams
The bedrock
that frames
the valley
is primarily
Butte quartz
monzonite
CONTINTENTAL
DIVIDE
The valley is drained by Blacktail, Little basin and SilverBow creeks and is filled with several hundred feet of alluvium and basin fill material derived from the surrounding bedrock - mountains that are composed mostly of Butte quartz monzonite. Not the fracture permeability f the bedrock. Groundwater flows through the bedrock and alluvium from the valley margins towards the streams.
MBMG – Ground-Water Assessment Program

9. Nitrate in the Summit Valley -Ground Water 64% (96 of 149)
(Analyses from GWIC)
Percentage of GW samples* above background (> 2.0 mg/L)
64% (96 of 149)
Percentage of GW samples* above the health standard
(10 mg/L)
15% (22 of 149)
This map shows the nitrate in wells not obviously impacted by mine waste (TDS < 700, ph > 6) and wells exclusive of the pole plant and CO tailings. The idea being to look at ambient conditions in the valley. For wells with more than one analysis the one with the highest NO3 concentration is plotted. The white dots represent concentration below 2 mg/L or background. The bright red dots represent wells with NO3 above the health standard of 10 mg/L and the light red dots represent wells with NO3 above background concentrations of 2 mg/L. circles are alluvial wells squares are bedrock wells.
-can see that a significant proportion of the wells have – 64% - have concentrations above background, and 15 % have concentration above the health standard.
Point out above background on the fringes, below background in middle of the basin.
MBMG – Ground-Water Assessment Program

10. depth below water table
Nitrate in the
Summit Valley
-Ground Water
Relationship of NO3 to
depth below water table
and aquifer type
depth below water (ft)
NO3 (mg/L)
It is also interesting to note that the NO3 is distributed fairly deep in the system. Typically NO3 decreases with depth, but we have wells in excess of 200 and 300 feet deep with concentrations above background and even above the health standard. The fractures in the bedrock provide a rapid and apparently deep transport of surface NO3 into the gw system.
*squares represent bedrock wells
MBMG – Ground-Water Assessment Program

11. Different Nitrate Sources?
Elevated NO3 is generally related to land use overlying the aquifer.
Different Nitrate Sources?
East Butte- sewered
Warren Heights - unsewered
Sewer district boundary
TDS < 700 mg/L
sewered
unsewered
Nitrates in ground water are generally related to the land use in the area overlying the aquifer. Looking at the distribution of NO3 in the ground water it is interesting that elevated NO3 occurs beneath sewered residential areas and the unsewered areas.
In areas like Warren hts. We have fairly high density development with bedrock at or near the surface, septic systems seem to be a logical source. However the east Butte area is sewered and which would suggest a different source of nitrate. Elevated Cl can also be an indicator of septic effluent, Looking at the relationship between NO3 and Cl the unsewered area shows a clear tight increasing trend. There is an increasing trend from the data in the sewered area however it is not as pronounced and there is a lot more scatter in the data … there is also more data…
Other potential sources include lawn fertilizers, and possible explosive residue related to mine blasting…
MBMG – Ground-Water Assessment Program

12. Source Identification ?
Stable Isotopes for
Source Identification ? 70
300 66 75 41 47
168 46
108 32
135 5 30
depth water enters
15N Sample Sites
Different Aquifer types
Basin-Fill & Bedrock
Different Land Use
Sewered & Unsewered
Different Depths
Shallow & Deep
Sewer district
boundary
In October, James Rose and Cam resample 15 wells in the valley, We chose wells with varying depths, ranging up to 300 feet and in different land use hydrogeological settings.
MBMG – Ground-Water Assessment Program

13. Nitrate in the Ground Water
October
8.9
6.7
10.0
2.8
3.6
2.6
2.4
9.5
12.5
5.5
45.5
8.5
5.7
6.9
2.3
NO3 mg/L
background
health standard
number of samples
The nitrate results show that concentrations in all the wells were above background and 3 were above the health standard, the shallow well in the pole plant take honors with a concentration of 45 mg/L. Couple of other things to note, in some areas we see higher conc. Along the up gradient margins and lower conc. In the middle of the valley may be indicative of dentification
Also this well when it was sampled in the spring of 2001 had a conc. Of 10 mg/L in the fall it was 2.6 mg/l, the results from the other wells are generally consistent with previous sampling events.
MBMG – Ground-Water Assessment Program

14. 15N of Ground Water 15N per mil fertilizers septic/manure
The results from the 15N sampling are pretty consistent, most are between 8 and 11 per mil.
It is interesting to note that the lighter values occur around the the margins here and the more enriched values are generally found in the middle of the basin which sort of reflects the NO3 conc. Distribution and may be indicative of denitification … but that may be a tad speculative.
However the most significant aspect of the results is that they fall within the expected range of NO3 derived from animal waste rather than fertilizer—except for this sample from the pole plant.
15N per mil
MBMG – Ground-Water Assessment Program

15. Typical Ranges of 18O and 15N
of Nitrate
(modified from Kendall and Aravena, 2000)
NO3 fertilizer
NH4 in fertilizer
and rain
soil N
manure and septic
18O
Plotting O18 and N-15 together see the results are clustered pretty tight, except for the pole plant sample. When compared to the expected range of values fertilizers, we’re not in there, the results are close to what might be expected from soil organic N, but again given our setting I think that soil-N is a likely source. When compared to the expected range for NO3 derived from manure or septic waste we’re right in there.
15N
MBMG – Ground-Water Assessment Program

16. Summit Valley -Surface Water
Sampling Sites *
WWTP
Thompson Park
MBMG – Ground-Water Assessment Program

17. Nitrate in the Summit Valley -Surface Water
November 2001 *
WWTP
River Miles From Thompson Park
Thompson Park
MBMG – Ground-Water Assessment Program

18. Nitrate in the Summit Valley -Surface Water
May 2002 *
WWTP
River Miles From Thompson Park
Thompson Park
MBMG – Ground-Water Assessment Program

19. Summary/Conclusions
Nitrate concentrations in the Clark Fork Basin ground water are generally low
Elevated NO3 occurs in the Summit valley GW
shallow/deep - alluvium/bedrock - sewered/unsewered
15 N and 18O suggest septic/manure source
different land use areas have same signature
**pole plant unique
Elevated NO3 occurs in the Summit valley SW
NO3 > 1 mg/L below septics and urban area
Ground water/surface water connection
dilution from WWTP ?
To sum up: The ground water in the Summit valley seems to have more than its fair share of NO3. It’s shallow and deep, in the alluvium and the bedrock, and its below sewered and unsewered areas.
Limited sampling of isotopes of N and O suggest a similar or related source. There was no difference in the isotopic signature beneath sewered or unsewered areas. The one sample from the pole plant was unique from the others – and certainly suggest a different source.
There also seems to be generous portions of NO3 in our surface water, base flow sampling last fall showed concs. above 1 mg/l from from 4 mile rd on Blacktail crk down to the Pole Plant and SilverBow Creek, it certainly suggests elevated concentrations that we are seeing in the gw are making their way into the surface water.
However it was interesting to note that below the WWTP NO3 conc in Silver Bow Crk dropped to ND
What’s the source in the sewered areas?
East Butte?
Do concentrations fluctuate seasonally?
Ground water
Septics constant source
Denitrification?
Surface water
Run off
Variations in WWTP discharge
What are the downstream impacts?
MBMG – Ground-Water Assessment Program