An Investigation of Techniques to
Predict and Quantify Stormwater
Chemical Concentrations in a Karst
Aquifer System
Rachel Grand
11 December 2003
Karst aquifers are notably different from
other types of aquifers.
Karst systems are characterized by highly soluble
bedrock, generally either limestone or dolomite.
As such, the aquifer has open-conduit flow, from
fractures, as well as dissolution features, such as
sinkholes and caves.
These aquifers also have diffuse flow.
Figure 1: Karst aquifer cross section. From
http://www.forester.net/images/sw0111_49.gif
Because of the open, fractured nature of
karst, surface contamination can impact
these aquifers. Understanding and
quantifying this surface contamination is
important for a variety of water-quality
issues.
Sources and Paths of Contamination in a
Karst Aquifer
Figure 2: Contamination in a karst aquifer. From
http://www.dyetracing.com/karst/ka01013.html
Techniques for Predicting Nitrate
Concentration through a Storm Event
Visual Inspection of storm hydrograph
Multiple Regression using stage and SC as a
proxy for nitrates
Step Multiple Regression using stage and SC
as a proxy for nitrates
Multiple Regression
Single equation generated to model nitrate
behavior
Stage and SC are the variables
Nitrates are the response
To be an effective technique, p-values must be
below -level (generally 0.05)
R-squared values should be high enough to
explain majority of variability
Step Multiple Regression
Sort stage data and plot versus nitrates
Identify obvious slope changes
Stafford Spring, AR
6
step 2
16.70-18. 54
step 1
9. 00-16.70
5
step 3
18.54-73. 00
NO3-N
4
3
2
1
0
0
10
20
30
40
Stage
50
60
70
80
Step Multiple Regression, continued
After identifying steps, generate a multiple
regression equation for each step
P-values must be below -level
R-squared values must explain a majority of
the variability
Plot equations and compare to actual nitrate
levels
Stafford Spring Simulated vs. Actual Nitrate
Concentrations
12
10
step 1
NO3
8
step 2
6
Step 3
4
Nitrate Concentration
2
0
Time (min)
Data from Peterson, Davis and Brahana, 2000.
Stafford Spring NO3 Concentrations
6
NO3 (mg/L)
5
4
Simulated NO3 using
MR
3
Measured NO3
2
1
0
time
Multiple Regression Plot (no steps)
Data from Peterson, Davis and Brahana, 2000.
Step Analysis of Millstone Spring, KY
Millstone Spring, KY
11000
outlier?
NO3
10000
Step 2
101.3-105.8
Step 3
105.8-111.9
9000
Step 1
95.3-101.3
8000
7000
6000
95
100
105
Stage
110
115
Simulated and Measured NO3 Millstone Spring
11000
NO3
10000
9000
8000
7000
6000
171
173
175
177
time (julian date)
179
measured N03
simulated NO3
Multiple Regression (no steps), Millstone Spring, KY
SC
Simulated Nitrates Using Step Regression
30000
25000
20000
15000
10000
5000
0
-5000172
-10000
-15000
-20000
174
176
178
180
step 1
step 2
time (julian date)
Step Regression, Millstone Spring, KY
step 3
NO3
Why Did These Techniques Not Work?
A larger data set may produce more reliable regression equations.
 Determining what makes an obvious slope change is not a quantifiable
evaluation, and is subject to individual interpretation.
Peterson, Davis and Brahana conclude that the step regression methods
work best for springs fed primarily by diffuse flow (2000, p. 61).
It is quite possible that Millstone Spring is supplied by conduit flow
instead.
It may be that the aquifer must be profoundly impacted (by intensive
agricultural use, for example) to be predictable using this model.
Works Cited
Croft, A., 2003, Introduction to Karst Environmental Problems,
http://www.dyetracing.com/karst/ka01013.html
Forester Communications, 2003, Karst Cross-Section,
http://www.forester.net/images/sw0111_49.gif
Peterson, E.W., Davis, R.K. and Brahana, J.V., 2000, The use of Regression
Analysis to Predict Nitrate-Nitrogen Concentrations in Springs of Northwest
Arkansas, in Sasowsky, I.D. and Wicks, C.M. (eds), Groundwater Flow and
Contaminant Transport in Carbonate Aquifers, A.A. Balkema, Rotterdam, p.4363.
In addition to the sources listed below, the statistical programs MINITAB, PSI-Plot
and Excel were used in the analysis of the data.
Dr. Dorothy Vespers provided the chemical data for Millstone Spring.