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 LimitAlkalinity 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 NATDS SWQMPb 10.0 mg l-1
TSS SM 2540 D 4.0 mg l-1
Turbidity SWQMPb NAArsenic (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)