Distribution of Nitrate in Ground Water Under Three Unsewered Subdivisions

Erin P. Eid

Mike Trojan

Jim Stockinger

Jennifer Maloney

Minnesota Pollution Control Agency

Ground Water Monitoring and Assessment Program

Why conduct a Septic System study?

• Assess water quality in unsewered areas

• Few studies comparing water quality between different unsewered subdivisions

• Limited information of spatial distribution of nitrate in unsewered subdivisions

• Determine risk to ground water and surface water

• Limited information for water and city planners

OUTLINE

• Study Design

• Results

• Conclusions

• Future Studies

STUDY DESIGN

• Objectives

• Variables

• Sampling Design

• Study Area

• Parameters

Objectives

• Determine the spatial distribution of nitrate in ground water under three unsewered subdivisions

• Identify factors affecting the distribution of nitrate under the three subdivisions

Variables

• Density and age of septic system and subdivision

• Aquifer hydrogeology and geochemistry

• Performance of septic system

• Depth to ground water

Sampling Design

• One-time sampling events using shallow geoprobe and deeper domestic well samples

• Collection of soils for textural analysis

Geoprobe and Domestic Well Cross-Section

Cross-section showing geoprobe (G) and domestic well (W) sampling locations through an aquifer

Depth

(m

)

Distance (m)

10

20

30

0

40

0 500 1000 1500 2000

G GWW

G G G GWWW

WWW

W

Site Criteria

• Sandy Soils

• Sand and Gravel Aquifer

• Less than one acre lot size

• Aquifers vulnerable to contamination

• Results applicable to other areas

• Local cooperation

Site Locations

• Baxter Area– sampled 37 domestic wells

– geoprobed 24 sites

• St. Joseph Area– sampled 23 domestic wells

– geoprobed 7 sites

• Lakeland Area– sampled 44 domestic wells

– geoprobed 9 sites

#Y

#Y#

#Y

##Twin Cities

#

Baxter

#

Lakeland

Pleasant Acres

Study Area - Baxter• Urban Area of 40,000

people• Considerable land use

changes last 10 years• Surficial geology consists of

outwash deposits of sand and gravelly sand

• Shallow sand and gravel aquifer with 15 feet of land surface

#Y#Y

#Y#Y #Y

#Y

#Y#Y

#Y

#Y#Y

#Y#Y

#Y#Y

#Y

#Y

#Y

#S

#S

#S

#S

#S

#S

#S

#S

#S

#S

#S

#S

#S

#S

#S

#S

#S

#S

#S

#S

#S

#S

#S

#S

#S

#S

#S

#S

#S

#S

#S

#S

#S

#S%U

%U%U%U

%U%U

%U

White SandLake

Sand LakeLower Wipple Lake

Clearwater

Hig

hway

371

Highland

Mapleton

#S

#Y%U

Domestic WellGeoprobeISTS Investigation

Study Area - Lakeland• Community of approximately 5,000

people• Has not experienced significant growth

in last 10 years• Surficial geology consists of coarse sand,

gravel, and gravelly sand terrace deposits• Depth of bedrock varies between 0 and

60 feet• Unconfined sand and gravel aquifer

found at 45 to 80 feet below land surface

#S

#Y G eop robe

D om estic W ell

#S#S#S#S#S#S#S#S #S#S#S #S#S#S#S #S#S#S

#S#S

#S

#S

#S #S

#S#S#S#S#S#S#S#S

#Y#Y #Y

#Y

#Y

#Y

#YS T

. CR

OIX

RIV

ER

I 94

St. C

roix

Tra

il

Study Area - Pleasant Acres

• Located near metropolitan area of 100,000 people

• Considerable land use changes within last 10 years

• Surficial geology consists of primarily outwash deposits of sand and gravelly sand

• Surficial sand and gravel aquifer located between 10 and 25 feet of land surface

#Y #Y

#Y

#Y

#Y

#Y

#Y#Y

#Y

#Y#S

#S

#S

#S

#S

#S

#S #S

#S

#S

#S

#S

#S

#S

#S

#S

#S

#S Domestic Well#Y Geoprobe

Parameters

• Field measurements:– oxidation-reduction potential, temperature, pH, specific

conductance, dissolved oxygen, and water level

• Laboratory Analysis:– Major cations and anions– dissolved organic carbon– ammonia- and Kjeldahl-nitrogen

RESULTS

Nitrate concentrations at the water table did not differ between the three areas

6.4 6.4 4.30

2

4

6

8

Nit

rate

(p

pm

)

Lakeland Baxter Pleasant Acres

Lakeland: Nitrate concentrations did not decrease with depth

0.1

1

10

0 50 100 150

Depth (feet)

pp

m o

r vo

lts

Nitrate Dissolved oxygen Eh

600

650

700

750

800

0 200 400 600 800 1000 1200

Lakeland Cross-section

Sand and Gravel

? Bedrock ?

Clay

Baxter: Nitrate concentrations decreased rapidly in the upper 20 feet of the surficial

aquifer

0.01

0.1

1

10

0 10 20 30 40 50 60

Depth (feet)

pp

m o

r vo

lts

Nitrate Dissolved oxygen Eh

Pleasant Acres: Nitrate concentrations decreased abruptly at about 25 feet

0.1

1

10

0 10 20 30 40 50 60

Depth (feet)

pp

m o

r vo

lts

Nitrate Dissolved oxygen Eh

800

850

900

950

1000

1050

1100

1150

1200

300 400 500 600 700 800 900 1000

Pleasant Acres Cross-section

SandClay

Sand

? Bedrock ?

The depth to which each aquifer is vulnerable to nitrate contamination varied

between the three areas

05

10152025

0 50 100 150

Depth (feet)

Nit

rate

(p

pm

)

Lakeland Baxter Pleasant Acres

Possible reasons for the differences with depth?

• Lower rates of denitrification in Lakeland

• Fractured bedrock flow in Lakeland

• Higher aquifer hydraulic conductivity in Baxter may move nitrate more deeply than in Pleasant Acres

• Pleasant Acres has well-defined geochemical zones

• Differences in effluent discharge

12 to 25 percent of samples collected at the water table exceeded the MCL of 10 ppm

17 25 12.5

0

5

10

15

20

25

Per

cen

t

Lakeland Baxter Pleasant Acres

Lakeland

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## #

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< 2 ppm

2 - 5 ppm

5 - 10 ppm

> 10 ppm

Pleasant Acres

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#

#

#

#

#

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#

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# #

2 - 5 ppm

< 2 ppm

5 - 10 ppm

> 10 ppm

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#

#

#

#

#

#

#

#

#

#

#

#

< 2 ppm

2 - 5 ppm

5 - 10 ppm

> 10 ppm

Baxter

• Nitrate concentrations at the water table under unsewered developments are about 4 to 6 ppm

• The depth of nitrate impacts from septic systems is controlled by aquifer geochemistry and hydrology

• In unsewered areas small lot size and the age of the development are factors in elevated nitrate levels

Conclusions

Future Studies / Use of Data

• Septic System studies in additional communities

• Effects of septic systems on sensitive surface water

• Relationship between septic systems, lot size, and age of development

• Computer simulations for prediction of effects of septic systems on ground water quality in other communities

http://www.pca.state.mn.us/water/

groundwater/gwmap