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Madeline SchreiberJonathan Gerst Heather ScottDepartment of Geosciences,Virginia Tech

Wil OrndorffVirginia Department of Conservation & Recreation

Benjamin SchwartzDepartment of Biology,Texas State University

Dan DoctorUS Geological SurveyReston VA

Recharge in karst systems: Implications for nutrient management

AcknowledgementsFunding from VWRRC, CCV, USGS‐NIWR. Field assistance:  Tom Malabad, Nathan Farrar, Ariel Brown, Andrea Futrell, VPI Grotto. Lab assistance:  Anna Hardy, Yinka Oyewumi, Athena Tilley

Recharge in karst systems

• How are karst aquifers recharged?

– diffuse infiltration through overlying soil/BR

– focused infiltration through fractures

– streams

• How do recharge mechanisms and timing affect introduction of surface contaminants to aquifer system?

• Two case studies in karst: Kentland Farm and James Cave

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Underlain by Cambrian ElbrookFm. dolostones and limestones

Sinkholes are subtle features with no bedrock exposed at the surface. No known caves.

Soil-covered (mantled) karst

Fluvial terrace deposits are 3 to >12m thick and are highly heterogeneous

Sinkholes at Kentland Farm (PhD research by B. Schwartz)

Images from B. Schwartz

Sinkhole #1

Sinkhole #5

Sinkhole #1

B B’

A A’

10 m

10 m

Sinkhole #5

A

A’

B’

B

C

C’

C’C

Approximate bedrock surface

50m

50m

VA

a

c

b

5.8 m

0.6 m

New River

Slide from B. Schwartz

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How does infiltration and recharge occur in soil-filled sinkholes?

Methods: Soil Science + hydrology + geophysics

Subject of B. Schwartz PhD at Virginia Tech

• Use TDR to measure soil moisture

• Calibrate measured to actual moisture (TDR does not measure 'actual' moisture!)

• Use ERT to measure electrical resistivity

• Calibrate ERT to soil moisture

• Use ERT to measure soil moisture

• Use differential ERT to get 2D picture of soil moisture, and to measure changes in soil moisture over time.

Soil moisture: Actual vs. TDR measured

Sample soil during augering, measure moisture in lab

Corresponding TDR readings

Data comparison

-25

-20

-15

-10

-5

0

0 20 40 60 80% Moisture by volume

De

pth

[ft

]

TDR moisture

Field moisture

Predicted moisture

Calibrate TDR measured to actual using multiple linear regression, includes clay %, soluble salts data => can use TDR to measure changes over timeSlide from B. Schwartz

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Electrical Resistivity Tomography (ERT)

http://www.komex.com/Geophysics/resistivity.stm

Measure subsurface electrical properties

Apparent resistivity values converted to modeled resistivity values via an iterative inverse modeling process

Slide from B. Schwartz

8

Differential Electrical Resistivity Tomography (DERT): Look at changes in resistivity over time

Time 1

Time 2

Difference

Slide from B. Schwartz

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9

Differential Electrical Resistivity Tomography (DERT)Seeing the changes in 2-D

Cumulative rainfall in inches

0

4

8

12

0 40 80 120 160

Days since first DERT profile

Inc

he

s

Slide from B. Schwartz

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Differential Electrical Resistivity Tomography (DERT)Seeing the changes in 2-D

Cumulative rainfall in inches

0

4

8

12

16

0 30 60 90 120Days since first DERT profile

Inch

es

Slide from B. Schwartz

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11

9-12-05 calibrated TDR readings

ERT dataTDR moisture data

Physical and chemical parameters

Statistical modeling to convert ERT data into moisture values

Integrate TDR and DERT

Slide from B. Schwartz

143

150

164

170

179

228

235

261

249

282

May 23, 2006

October 9, 2006

meters

meters

metersmeters

Rain events: d 173‐d 187

Using DERT to visualize changes in soil moisture over time --can see recharge happening!

Schwartz and Schreiber, 2008

Sinkhole 1

Recharge is mostly diffuse (and slow!) but some pooling at bottom and some regions of flanks

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Implications for nutrient management in karst regions with sinkholes

Not all sinkholes are alike!  End members and lots of variation in between.

open pore throats:  direct connection to aquifer

thick mantles:  diffuse, slow infiltration

this study:  mostly diffuse infiltration, but some evidence for enhanced infiltration at sinkhole flanks and bottom during heavy rain events.

Q: would 50 ft setback for nutrient application be protective of groundwater quality? 

Need for more scientific study on sinkholes and how they act to transmit/store water and solutes

cow carcass

What is the timing and quality of recharge to karst aquifers?

Subject of J. Gerst MS Thesis at VT

Study site: James Cave (Pulaski Co).

Cambrian Elbrook Fm. dolostones and limestones

fractured with low matrix porosity

typical of SV karst

Cave entrance in sinkhole bottom.

Rolling hills and sinkholes, 0.5 to >2m thick soils, scattered bedrock outcrops

Timing and quality of recharge: James Cave

Cow pasture now, but prior to 2008, fertilizer applied for hay.

NPK fertilizer still used in adjacent fields, upgradient from cave entrance

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We are monitoring precipitation, soil water, cave drips and the cave stream.

Survey of monitoring stations in James Cave (Image from Tom Malabad).

MS1 and MS2 are 15m apart, 50m and 65m west of cave entrance, respectively.MS3 is ~60 m east of cave entrance.

MS 1

MS 2

MS 3

CAVE ENTRANCE

STREAM ACCESS

Drip monitoring involves capturing drips on a tarp and funneling through a rain gauge

Drip site #1 = 7.9 m deep

Drip site #2 = 10.6 m deep

Drip site #3 = 7.1 m deep

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Recharge via drips occurs winter to mid-spring, varies year to year

Recharge period 2008

Recharge period 2009

Recharge period 2010

Nitrate concentrations in drips and stream variable; stream nitrate is below MCL.

Key points:1) Spatial variability!Site 3 has highest nitrate. This is the most upland site! 

2) MS1 closest to sinkhole bottom, would expect highest nitrate.

3) Nitrate in stream is below MCL.

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Implications for nutrient management in karst regions with caves

Recharge (drips) occurs during winter-early spring. Actual timing varies year to year. 2011 recharge started late.

Q: Should the timing of recharge influence timing of fertilizer application?

The most upland drip site (175 ft from sinkhole bottom) has highest nitrate concentrations.

Q: Would a 50 ft setback for nutrient application improve this?

Need for field experiments with different application practices -- location and timing! Characterize system first, then see how it changes