Christine Miller, Tammy Swinney, Erinn Zindt, Torie Haraldson

MT Section AWRA Conference, October 10th, 2014

Gallatin Groundwater Project: Assessing Cumulative

Impacts to Groundwater in High Density

Septic System Areas

Background

• Between 1990 and 2009, the population of Gallatin County, Montana increased by approximately 79% (U.S. Census Bureau, 2007)

• Some subdivision areas have been developed with individual septic systems for each home

• Wastewater effluent from areas of high septic system density can negatively impact groundwater and surface water quality

Photos: A. English

Background • ~13,350 active septic systems discharging about 4 million gallons per day

of effluent into groundwater in Gallatin County (English, Assessment of Current

Wastewater Treatment and Disposal in Gallatin County, 2010)

• All treated wastewater from these septic systems as well as from municipal and public systems in Gallatin County is discharged to either groundwater, surface water, or applied to the land surface

• Many residents in the Gallatin Valley rely on groundwater as their drinking water source

Project Goal

• In the Gallatin Local Water Quality District, assess whether individual septic systems and public sewage systems in high density development areas are negatively impacting water quality.

1) Are areas of high density septic systems and public sewage systems measurably and negatively impacting groundwater quality?

2) Have historical nitrate levels increased down-gradient of subdivisions characterized by high density septic systems and public sewage systems?

3) Are nitrate levels trending upward for public water supplies down-gradient of developed areas?

Project Questions

Groundwater flow is generally to the northwest in the Gallatin Valley

Wells sampled in subdivision focus

areas are generally shallow (<100’) and in alluvial aquifers

Methods

- Groundwater sampling of eight subdivision areas for various analytes

- Nutrients (nitrate, orthophosphate)

- Wastewater tracers (boron, chloride, specific conductivity)

- Nitrate isotopes – can help identify source

- Gathered historical nitrate data

- Gathered data from nitrate sensitivity analyses from non-degradation reports

- Gathered Public Water Supply nitrate data

Results – Nitrate-N in 2013 Samples

< 2 mg/L 47.8%

2-4.99 mg/L 32.8%

5-10 mg/L 15.9%

>10 mg/L 3.5%

Elevated nitrate is not a widespread problem in the wells sampled

0

5

10

15

20

25

30

(mg

/L)

Mean Nitrate-N

Mean Chloride

Results - Mean Nitrate-N and Chloride for Subdivision Focus Areas

Results - Mean Orthophosphate-P and Boron for Subdivision Focus Areas

0.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

(mg

/L)

Mean Orthophosphate

Mean Boron

Results – Nitrate Isotopes

-20

-15

-10

-5

0

-5 5 15 25

δ1

8O

-NO

3-

δ15N-NO3-

High K, River Rock

Hyalite Heights/Lazy TH

Western Drive

Mountain View

Middle Creek

Jackrabbit

Baxter/Wylie

Wheatland Hills Manure and Septic Waste

Fertilizer and Precipitation NH4

+

Soil NH4

+

One subdivision area with elevated nitrate is located in an area of higher

soil organic matter

Results – Historical vs. 2013 Nitrate

1.60 2.58

9.37

5.88 5.30 3.20

41.00

4.97

2.11 2.90

17.40

7.27

12.80

6.16

10.10

6.40

0

5

10

15

20

25

30

35

40

45

Max

Nit

rate

-N (

mg

/L)

Max Historical Nitrate-N

Max 2013 Nitrate-N

Non-Degradation and Nitrate

1.08 1.12

1.97

1.41 1.29

1.87

3.10

2.65

3.28

2.61

3.35

5.20

1.41 1.70

3.48

2.07

2.84

4.93

0

1

2

3

4

5

6

Nit

rate

-N (

mg

/L)

Mean Background Non-Degradation Nitrate

Mean Non-Degradation Expected Nitrate

Mean 2013 Nitrate

Results – Nitrate Trends in Public Water Supplies near Subdivision Focus Areas

Example:

Slope of linear trend lines compared (increasing or decreasing )

0

2

4

May-70 Nov-75 Apr-81 Oct-86 Apr-92 Sep-97 Mar-03 Sep-08 Mar-14

Nit

rate

-N (

mg

/L)

Nitrate Trends in PWS Near Middle Creek Focus Area Four Corners Baptist Church

Garden Center Subdivision + -

Conclusions

• Widespread cumulative effects from septic systems on groundwater currently not observed

• Localized areas of concern

• Nitrate source: septic/animal waste and/or soils, fertilizer in some areas

• Increasing nitrate levels in groundwater, expected to continue

• Some public water supplies show an increase in nitrate

Recommendations

• Educational Efforts

• Maintain septic systems (pumping, inspections)

• Appropriately fertilize lawns and gardens – (Minimize additional nutrient additions to groundwater)

• Avoid excessive watering (reduce nitrate leaching)

• Continue to encourage community water and wastewater systems

• Especially in Source Water Protection (SWP) areas

• Regular monitoring required

• Problems pinpointed and more easily remedied compared to diffuse contamination from array of septics

Recommendations Cont’d

• Encourage installation and sampling of down-gradient monitoring wells for new developments • Early identification of water quality problems or wastewater

treatment system problems

• Long term water quality data sets created

• Annual testing of domestic drinking water • Protect health of Gallatin Valley residents consuming water from

private domestic wells

• Long term water quality data sets created

Thank you to: – Montana DEQ 319 Grant Funding

– Gallatin County Planning Department

– Gallatin City-County Environmental Health Services (EHS)

– Gallatin County GIS Department

– Homeowners and residents who allowed GLWQD staff to

sample their domestic wells – Gallatin County/MSU Extension

Thank you!

Questions?