Transcript Applications of Hydrogeologic settings in Groundwater vulnerability mapping in LaGrange County, Indiana, USA Solomon A.

Applications of Hydrogeologic
settings in Groundwater
vulnerability mapping
in LaGrange County, Indiana,
USA
Solomon A. Isiorho, PhD
Dept. of Geosciences
Indiana University - Purdue University
Fort Wayne, IN 46805
[email protected]
Outline of Presentation

Problems

Study area

Hydrogeologic Setting

Methods/Discussions

Conclusions
Problems

More than 60% depend on Ground water (100% in the study
area)

Three-dimensional detail geologic mapping…..to support
informed decisions on land use, water resources development
and protection, etc, is lacking

USEPA (1992)…maps did not correlate well with water quality
analysis performed in the national Survey for Pesticides in
Drinking Water

Increase in human population (~18% in 10 yrs (9% State wide 6.2 m))

Contaminated potable water supply

Several methods for assessing…produce different maps
Study Area
North America
LaGrange
USA
Map of Indiana
LaGrange County, Indiana
Hydrogeologic Setting
Unconsolidated Aquifer
Groundwater flow direction
(after Clendon & Beaty, 1987)
Groundwater flow direction
(from T. Fleming, 1996)
Methods

Examination of existing vulnerability map

DRASTIC

Collection of nitrate level data

Compare nitrate levels distribution with
DRASTIC groundwater vulnerability map
DRASTIC
Depth to ground water,
Recharge rate,
Aquifer media,
Soil media,
Topography,
Impact of the vadose zone and
Conductivity of the aquifer (DRASTIC).
Assumptions in DRASTIC
The developers of DRASTIC noted that
1. Method does not replace site specific investigation
2. A measure of relative groundwater vulnerability……
one of many criteria used in decision making
3. Erroneous or inaccurate data entered may affect reliability of the results
4 Major Assumptions
Uniform conditions within the subsurface environment
Contamination reaches groundwater by precipitation
Contaminant has the mobility of water
Area of the study is greater than 100 acres
DRASTIC
Criteria
Data Source
Depth to groundwater
Well logs
Net recharge
Water resource Reports
Aquifer media
Hydro-geologic report
Soil media
Soil Survey State Soils Geographic Database
Topography
Published Topographic maps
Impact of vadose zone media
Published Geologic reports
K of an aquifer
Published Hydrogeologic reports.
USGS and IDNR
Indiana 30 year Average
USGS and IDNR
Vulnerability Map derived from DRASTIC
(from Cooper, 1996)
Soil Map of LaGrange County
Vulnerability Map
DRASTIC map
SOIL map
Collection of Nitrate level Data

Survey questionnaires 400

800 water bottles….507 nitrate data points…Health County
personnel.

Total of 1010 data points (503 from existing points)

Thirty one percent (311) high nitrate levels

But only 15% (150) are above 10ppm

90% wells with high nitrate <15 m

Regression analysis…animal waste/depth significant with respect to
nitrate level (F=-5.26, p<0.008)
Nitrate Level Distribution Map
(Red>10 ppm; Green =2-10ppm,Blue <2ppm; Black=0)
LaGrange County
showing Nitrate levels
above 10ppm
Nitrate
levels
Comparing Nitrate levels with DRASTIC Vulnerability Map
Why no perfect match between DRASTIC and
Nitrate level maps?
Hydrogeologic Setting/Landuse
(from Summit Risk Inc)

Good correlation between nitrate detect and
pesticides…implication for other contaminants

(from ???)
Effects of abandoned wells
and impact of other land use
(After Petty, 1996)
Conclusions

Hydrogeologic mapping is important in the determination
of groundwater vulnerability in any given area.

Different methods yield slightly different groundwater
vulnerability maps and would change with time.

Vulnerability maps produced for any area should be
regarded as a working document. Future land use,
including well construction and abandonment, would
have great impact on such vulnerability maps.