Effects of Suburban Development on Shallow Groundwater Quality

Transcription

1 Effects of Suburban Development on Shallow Groundwater Quality J O S H K A S P E R, D N R E C J U D Y D E N V E R, U S G S J O A N N A Y O R K, U D F U N D I N G F R O M T H E D N R E C, U S G S, A N D C E N T E R F O R T H E I N L A N D B A Y S D E C E M B E R 7,

2 Shallow water table and a thick, sandy surficial aquifer susceptible to nitrogen (N) contamination Groundwater is the major contributor of N to streams and the Inland Bays, which have a significant problem with eutrophication The effects of agriculture, currently the major land use, on groundwater quality are well understood As land use changes from agricultural (and forested) to suburban, the effects of suburban land use on water quality will become more important especially with respect to concentrations of nitrate in water reaching the Inland Bays Problem

3 Objectives Characterize groundwater quality beneath a suburban development with septic systems Compare concentrations of nitrate and other waterquality constituents in water affected by suburban development to natural background water quality and water affected by agricultural land use. Identify hydrochemical patterns that can be used as tools for understanding the source of nitrate in groundwater. Implications for future water quality

4 Study Sites Agricultural Study Site Hopkins Prong Suburban Study Site A A' Guinea Creek EXPLANATION ( Well or well cluster location Land Use Suburban Forestland km A A' Line of profile (see Figure 8) Agriculture Rangeland Wetlands Water

5 Suburban Study Site Suburban Study Site A A' Guinea Creek A EXPLANATION Land Use Permanant Private Well Suburban Forestland Temporary Geoprobe Well Agriculture Wetlands A' Line of profile Rangeland Water km

6 Elevation, in ft msl Hydrogeology 10 0 Pi Columbia aquifer. -20 Gamma Temporary well Primarily quartz sand. Aquifer thickness variable, but ~100 ft thick at gamma-ray log location. Upper 80 ft of aquifer characterized by this study Private well Columbia aquifer Confining bed -130 Pocomoke aquifer Gamma, in API-GR

7 Scope of Work Groundwater samples collected during Spring 2011 Samples were collected just below the water table (~10-15 ft) using a Geoprobe at 29 sites; at three sites samples were collected at multiple depths from ft. Samples were also collected from 22 domestic wells (25-80 ft deep; median depth 54 ft) All samples: field parameters; nutrients and major ions at NWQL; N and O isotopes of nitrate at UD.

8 Scope of Work (cont.) Water chemistry data collected for this project were compared to data from previous study at a nearby agricultural site (Fairmount) and to natural water chemistry Some problems with saltwater to work around (cut off most analyses at Cl>50 mg/l) Draft paper has been prepared and will be submitted to a journal for consideration.

9 Geoprobe well installation

10 Geoprobe well installation (cont.)

11 Sampler Sheath Stainless Steel Well Screen Bentonite Plug Turf grass Q Not To Scale Vadose Zone Drill Rod Water Table Saturated Zone Drive Point

12 Results Estimate of groundwater age

13 Results Nitrate as N Overall medians (mg/l) Temporary wells (2.7) Private wells (1.5) Agricultural wells (16.9) Nitrate in private wells <10 mg/l PMCL Nitrate >10 mg/l in two temporary wells Open lots had higher nitrate concentrations than forested lots

14 Results Nitrate as N N it r a t e a s N, in m g /L N it r a te R e s u lts fo r T e m p o r a r y G e o p r o b e W e lls >1 0 5 to to 5 <0.4

15 Results Nitrate as N N it r a te R e s u lts fo r P e r m a n a n t P r iv a t e W e lls N it r a t e a s N, in m g /L >1 0 5 to to 5 <0.4

16 SC 91 Nitrate 1.9 SC 118 Nitrate 6.4 Results Geochemical Signatures

17 Results Geochemical Signatures

18 Results - Iron 300 Dissolved iron most elevated in temporary Geoprobe wells May be related to iron in fertilizer

19 Isotopic values of N & O in NO 3 - indicated overlapping sources of N, with some indication of denitrification

20 High δ 15 N values corresponded with low nitrate concentrations, indicating denitrification Turf fertilization

21 Characterization of Groundwater Beneath Suburban Development Shallow groundwater affected by turf management (of varying degrees) Median nitrate 2.7 mg/l as N (< ) Nitrate higher beneath open lots than beneath forested lots Water types variable due to differences in turf management Ca-Mg-NO3-Cl water type in some areas resembles agricultural signatures, but with lower concentrations of fertilizer components (usually), higher dissolved iron, and proportionally more sulfate Deeper groundwater from domestic wells Median nitrate 1.5 mg/l as N ( ) Nitrate higher beneath open lots than beneath forested lots Mostly a Na-Cl-NO3 water type; some effects from septic system effluent and maybe fertilizer ( mixed sources) Natural Na-Cl-HCO3 water type (from upgradient forested recharge area) identified in 4 domestic wells. N & O isotopes of NO3 apparently most useful for looking at denitrification in areas with these land-use impacts

22 Conclusions As suburban development increases, lower concentrations of nitrate in groundwater affected by suburban land uses should have a positive impact on nutrient concentrations in surface water, however: The effects of fertilization of turf can be as great as agricultural applications of fertilizer There are other contaminants associated with development that may increase (e.g., VOCs, Pesticides, Pharmaceuticals) These results for the Coastal Plain where sediments are primarily quartz sand and anthropogenic impacts commonly dominate major-ion chemistry are transferable to areas with similar conditions

23 d 18 O ( ) NO 3 in precipitation desert NO 3 deposits NO 3 fertilizer 10 0 NH 4 in fertilizer and rain soil N manure and septic waste d 15 N ( )

24 d 18 O-NO 3 - ( ) 20 natural septic NH 4 in fertilizer and rain soil N manure and septic waste managed turf 1999 Farmount 2011 Farimount d 15 N-NO 3 - ( )

25 C h lo r id e R e s u lt s fo r T e m p o r a r y G e o p r o b e W e lls C h lo r id e, in m g /L > to to to to 2 5

26 C h lo r id e R e s u lt s fo r P e r m a n a n t P r iv a t e W e lls C h lo r id e, in m g /L > to to to to 2 5

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29 Results Dissolved Oxygen Groundwater was predominantly oxic (DO>0.5 mg/l) Notes: AG=Agricultural well TT=Temp. well (turf) TM=Temp. well (turf/forest) PT=Private well (turf) PM=Private well (turf/forest)

30 Results - Chloride Highest in temporary wells in eastern part of suburban study site (similar to SEC) High Cl samples (>50 mg/l) removed

31 Results Specific Electrical Conductance Highest in temporary wells in eastern part of suburban study site

32 Results Water Chemistry Na-Cl-HCO3 Na-Cl-HCO3 Na-Cl-NO3 Ca-Mg-NO3-Cl