Characterizing Storm Water Runoff from Natural Gas Well Sites

in North-Central Texas

Paul F. Hudak and David J. WachalDepartment of GeographyUniversity of North Texas

Introduction

* Annually, 30,000+ natural gas wellsdrilled in U.S.

* Potentially adverse to surface water quality

(sediment, hydrocarbons, metals)

* However, few studies have been conducted

Introduction (continued)

* The objective of this proposal is tocharacterize storm water runoff from

naturalgas well sites in north-central Texas

Background

* Adverse effects of “typical” constructionactivity well known

- Regulated by National Pollutant DischargeElimination System (NPDES), requiring erosioncontrol, storm water pollution prevention plans,site monitoring

Background (continued)

* Adverse effects of gas well construction not

well known

- Oil and gas field operations exempt fromfederal NPDES permitting requirements

Methods

* Collect storm water runoff from 3 natural gas well sites, 2 undisturbed reference sites in southwest Denton County

- Grand Prairie, montmorillonitic clays

- Average annual rainfall 99 cm, thunderstorms common

* ISCO flow-interval samplers: Up to 18 discrete 1000 ml samples

* 40 runoff events (total) at gas well sites, 10 at reference sites, over a two-year period

Methods (continued)

* Collect during rain events

* Metals, petroleum hydrocarbons, sediment

- Nonparametric statistics to compare differencesbetween gas well, reference sites

- Estimate annual sediment yields

Parameter Method Detection Limit

Alkalinity SM 2320 B 1.0 mg l-1

Calcium EPA 200.8 0.5 mg l-1

Chloride SM 4500 Cl (D) 0.15-10.0a mg l-1

Conductivity SWQMPb 10 µS m-1

Hardness SM 2340 C 1.0 mg l-1

pH SWQMPb NA

TDS SWQMPb 10.0 mg l-1

TSS SM 2540 D 4.0 mg l-1

Turbidity SWQMPb NA

Arsenic (As) EPA 200.8 0.01 mg l-1

Cadmium (Cd) EPA 200.8 0.001 mg l-1

Chromium (Cr) EPA 200.8 0.01 mg l-1

Copper (Cu) EPA 200.8 0.01 mg l-1

Iron (Fe) EPA 200.8 0.05 mg l-1

Lead (Pb) EPA 200.8 0.001 mg l-1

Manganese (Mn) EPA 200.8 0.01 mg l-1

Nickel (Ni) EPA 200.8 0.01 mg l-1

Zinc (Zn) EPA 200.8 0.05 mg l-1

TPH TCEQ 1005.3 5.0 mg l-1

BTEX EPA 2081 B 1.0 µg l-1c; 2.0 µg l-1d

a based on turbidity of sample; b Surface Water Quality Monitoring Procedures Manual (TCEQ, 1997) using Hydrolab, Y.S.I., or other similar meter; c for each Benzene, Toluene, Ethylbenzene, o-Xylene; d for each m-Xylene, p-Xylene

Methods (continued)Analytical Methods and Detection Limits

Parametera nb/nc p-value Ratiod

Alkalinity 40/10 <0.0001 6.9Chloride 40/10 0.0058 1.7Conductivity 40/10 <0.0483 1.2Hardness 40/10 <0.0001 3.2pH 40/10 <0.0001 1.2TDS 40/10 0.0561 1.2TSS 39/8 <0.0001 157.1Turbidity 37/9 <0.0001 42.5Calcium 36/8 <0.0001 8.0Iron 36/8 <0.0001 13.5Manganese 36/8 <0.0001 28.9Nickel 36/8 0.0027 3.2a Parameters containing more than 50% of storm event EMCs below detection limit (As, Cd, Cr, Cu, Pb) were not analyzed; b no. of gas well site samples; c no. of reference site samples; d ratio of median EMCs for reference and gas well sites.

Example Statistical Procedure (Preliminary Results):Wilcoxon Rank-Sum and Ratio of Median EMCs

Methods (continued)

* Also compare observations to drinking water standards and aquatic life criteria (the level of a pollutant or other measurable parameter that allows for protection of aquatic life)

Conclusion and Expected Benefits

* Study will show which pollutants are higher at gas drilling

sites compared to other construction sites

* Helps inform monitoring strategies

* Is beneficial to academic community, drillers,and regulatory agencies

Conclusion and Expected Benefits (continued)

* If pollutant levels are similar to, or higher than, other construction sites, this may signal a need for also

regulating gas well sites

* Future work could focus on best management practicesfor reducing pollutants from gas well sites similar toconstruction sites (regulated under NPDES)