An Investigation of Techniques to Predict and Quantify Stormwater Chemical Concentrations in a Karst...
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Transcript of An Investigation of Techniques to Predict and Quantify Stormwater Chemical Concentrations in a Karst...
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.
Figure 2: Contamination in a karst aquifer. From http://www.dyetracing.com/karst/ka01013.html
Sources and Paths of Contamination in a Karst Aquifer
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 RegressionSort stage data and plot versus nitrates
Identify obvious slope changes
Stage
0 10 20 30 40 50 60 70 80
NO
3-N
0
1
2
3
4
5
6Stafford Spring, AR
step 19.00-16.70
step 216.70-18.54
step 318.54-73.00
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
0
2
4
6
8
10
12
Time (min)
NO
3
step 1
step 2
Step 3
Nitrate Concentration
Data from Peterson, Davis and Brahana, 2000.
Stafford Spring NO3 Concentrations
0
1
2
3
4
5
6
time
NO
3 (m
g/L)
Simulated NO3 usingMR
Measured NO3
Multiple Regression Plot (no steps)
Data from Peterson, Davis and Brahana, 2000.
Stage
95 100 105 110 115
NO3
6000
7000
8000
9000
10000
11000Millstone Spring, KY
outlier?
Step 195.3-101.3
Step 2101.3-105.8
Step 3105.8-111.9
Step Analysis of Millstone Spring, KY
Simulated and Measured NO3 Millstone Spring
6000
7000
8000
9000
10000
11000
171 173 175 177 179
time (julian date)
NO
3
measured N03
simulated NO3
Multiple Regression (no steps), Millstone Spring, KY
Simulated Nitrates Using Step Regression
-20000-15000-10000-5000
05000
1000015000200002500030000
172 174 176 178 180
time (julian date)
SC
step 1
step 2
step 3
NO3
Step Regression, Millstone Spring, KY
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 CitedCroft, 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.43-63. 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.