Schafer

A Monte Carlo Model of Tongue River Water Quality: 

A Monte Carlo Model of Tongue River Water Quality William Schafer, Michael Bergstrom, & William Thompson Schafer Limited, Fidelity Exploration and Production, & Hydrometrics Presented at the First Annual CBNG Research, Monitoring, and Applications Conference, University of Wyoming, August 17-19, 2004

Overview: 

Overview Tongue River basin has seen substan- tial CBNG development since the late 1990’s Tongue River water quality and flow regime is very different than the Powder River owing to its source in the Big Horn Mountains Mean flow ~400 cfs, EC ~650 uS/cm, SAR ~ 1.0

CBNG Development Issues: 

CBNG Development Issues Irrigators and environmental groups have opposed CBNG development State of Montana promul- gated numeric standards to protect irrigation water quality in 2003

Tongue River Numeric Standards: 

Tongue River Numeric Standards

Fidelity Operations in Montana: 

Fidelity Operations in Montana Fidelity has operated the CX Field in Montana since 1999 Fidelity manages a portion of their produced water by direct discharge under a NPDES permit NPDES permit, currently being renewed, must address numeric standards, and according to State law, may be flow-based Fidelity’s existing permit is not flow-based, but was calculated to be protective at the 7Q10 flow of 39 cfs

Development of Tongue River Dynamic Mass Load Model: 

Development of Tongue River Dynamic Mass Load Model In order to assess effluent limits for the permit renewal, a water quality model of the Tongue River was needed A spreadsheet model was used Constituents modeled included EC, SAR, ammonia, and fluoride Model estimates assimilative capacity of Tongue River for direct discharge of produced water

Modeling Approach: 

Modeling Approach Develop mass load model Partition hydrograph into flow tiers Develop empirical relationship between ambient flow & chemistry Develop statistics for produced water chemistry Estimate maximum potential produced water discharge for each flow tier Validate model using observed Tongue River flow and chemistry Use mass load model to assess protectiveness of estimated CBNG discharge rates developed for each flow tier

Daily Flows at Stateline: 

Daily Flows at Stateline

Tongue River Flow Duration Curve: 

Tongue River Flow Duration Curve

Ambient water quality: 

Ambient water quality Water quality varies with flow Empirical model used to relate water quality and flow C when (Q<Qt) = Cbf C when (Q>Qt) = ((Qt Cbf)+((Q-Qt)*Cro))/Q Qt = 255 Cbf = 705 Cro = 153

Ambient flow: 

Ambient flow

Produced Water Characteristics: 

Produced Water Characteristics

Model Structure: 

Model Structure Model solves the following for each daily flow measurement: CRW = [(CTR*QTR)+(CPW*QPW)]/(QTR+QPW) The assimilative capacity (QPW) for each flow tier was calculated using a parametric approach

Predicted EC: 

Predicted EC

Predicted EC: 

Predicted EC

Predicted EC: 

Predicted EC

Model Validation: 

Model Validation Model-derived water quality was compared to actual water quality for a three year period of record (2000-2003) Model results showed that existing Fidelity permit has been protective of numeric water quality standards for EC and SAR, adopted in 2003

Tongue River and Produced Water Flow: 

Tongue River and Produced Water Flow

Estimated and Measured EC: 

Estimated and Measured EC

Estimated and Measured EC: 

Estimated and Measured EC

Correlation of Estimated and Measured EC & SAR: 

Correlation of Estimated and Measured EC & SAR

Assimilative Capacity of Tongue River for Produced Water: 

Assimilative Capacity of Tongue River for Produced Water

Instantaneous EC Criterion: 

Instantaneous EC Criterion

30-Day Average EC Criterion: 

30-Day Average EC Criterion

Instantaneous SAR Criterion: 

Instantaneous SAR Criterion

Chronic Ammonia Limit (Winter): 

Chronic Ammonia Limit (Winter)

Chronic Ammonia Limit (Summer): 

Chronic Ammonia Limit (Summer)

Conclusions: 

Conclusions The Tongue River dynamic mass load model uses Monte Carlo simulation techniques to evaluate mixing of Tongue River water and produced water. The model considers the flow-dependent chemical distribution of ion concentrations in upstream (ambient) Tongue River water and accounts for the random variation in the chemistry of produced water. Constituents modeled included EC, SAR, fluoride and ammonia.

Conclusions: 

Conclusions The model agrees with historic water quality monitoring data and shows that the current Fidelity MPDES discharge permit (MT-0030457), approved in 2000, has maintained downstream EC and SAR levels below criteria specified in the recently promulgated numeric standards for EC and SAR during the entire span of the permit

Conclusions: 

Conclusions Model results showed that the Tongue River can assimilate an average of over 8,200 gpm of CBNG water Ammonia was the most limiting factor Monitoring downstream of outfalls shows that ammonia is not conservative The assimilative capacity for fluoride and EC ranged from 14,000 to 18,000 gpm