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.