Application of Monte Carlo Methods to Perform Uncertainty and Sensitivity Analysis on Inverse WaterRock Reactions with NETPATH
Abstract
Methods were developed to quantify uncertainty and sensitivity for NETPATH inverse waterrock reaction models and to calculate dissolved inorganic carbon, carbon14 groundwater travel times. The NETPATH models calculate upgradient groundwater mixing fractions that produce the downgradient target water chemistry along with amounts of mineral phases that are either precipitated or dissolved. Carbon14 groundwater travel times are calculated based on the upgradient sourcewater fractions, carbonate mineral phase changes, and isotopic fractionation. Custom scripts and statistical code were developed for this study to facilitate modifying input parameters, running the NETPATH simulations, extracting relevant output, postprocessing the results, and producing graphs and summaries. The scripts read userspecified values for each constituent’s coefficient of variation, distribution, sensitivity parameter, maximum dissolution or precipitation amounts, and number of Monte Carlo simulations. Monte Carlo methods for analysis of parametric uncertainty assign a distribution to each uncertain variable, sample from those distributions, and evaluate the ensemble output. The uncertainty in input affected the variability of outputs, namely sourcewater mixing, phase dissolution and precipitation amounts, and carbon14 travel time. Although NETPATH may provide models that satisfy the constraints, it is up to the geochemist to determine whether the results are geochemically reasonable. Two example waterrock reaction models from previousmore »
 Authors:

 Desert Research Inst. (DRI), Reno, NV (United States)
 Publication Date:
 Research Org.:
 Desert Research Inst. (DRI), Reno, NV (United States)
 Sponsoring Org.:
 USDOE National Nuclear Security Administration (NNSA)
 OSTI Identifier:
 1258031
 Report Number(s):
 45267
DOE/NV/000093931
 DOE Contract Number:
 NA0000939
 Resource Type:
 Technical Report
 Country of Publication:
 United States
 Language:
 English
 Subject:
 54 ENVIRONMENTAL SCIENCES; Methods were developed to quantify uncertainty and sensitivity for NETPATH inverse waterrock reaction models and to calculate dissolved inorganic carbon, carbon14 groundwater travel times. The NETPATH models calculate upgradient groundwater mixing fractions that produce the downgradient target water chemistry along with amounts of mineral phases that are either precipitated or dissolved. Carbon14 groundwater travel times are calculated based on the upgradient sourcewater fractions, carbonate mineral phase changes, and isotopic fractionation. Custom scripts and statistical code were developed for this study to facilitate modifying input parameters, running the NETPATH simulations, extracting relevant output, postprocessing the results, and producing graphs and summaries. The scripts read userspecified values for each constituent’s coefficient of variation, distribution, sensitivity parameter, maximum dissolution or precipitation amounts, and number of Monte Carlo simulations. Monte Carlo methods for analysis of parametric uncertainty assign a distribution to each uncertain variable, sample from those distributions, and evaluate the ensemble output. The uncertainty in input affected the variability of outputs, namely sourcewater mixing, phase dissolution and precipitation amounts, and carbon14 travel time. Although NETPATH may provide models that satisfy the constraints, it is up to the geochemist to determine whether the results are geochemically reasonable. Two example waterrock reaction models from previous geochemical reports were considered in this study. Sensitivity analysis was also conducted to evaluate the change in output caused by a small change in input, one constituent at a time. Results were standardized to allow for sensitivity comparisons across all inputs, which results in a representative value for each scenario. The approach yielded insight into the uncertainty in waterrock reactions and travel times. For example, there was little variation in sourcewater fraction between the deterministic and Monte Carlo approaches, and therefore, little variation in travel times between approaches. Sensitivity analysis proved very useful for identifying the most important input constraints (dissolvedion concentrations), which can reveal the variables that have the most influence on sourcewater fractions and carbon14 travel times. Once these variables are determined, more focused effort can be applied to determining the proper distribution for each constraint. Second, Monte Carlo results for waterrock reaction modeling showed discrete and nonunique results. The NETPATH models provide the solutions that satisfy the constraints of upgradient and downgradient water chemistry. There can exist multiple, discrete solutions for any scenario and these discrete solutions cause grouping of results. As a result, the variability in output may not easily be represented by a single distribution or a mean and variance and care should be taken in the interpretation and reporting of results.
Citation Formats
McGraw, David, and Hershey, Ronald L. Application of Monte Carlo Methods to Perform Uncertainty and Sensitivity Analysis on Inverse WaterRock Reactions with NETPATH. United States: N. p., 2016.
Web. doi:10.2172/1258031.
McGraw, David, & Hershey, Ronald L. Application of Monte Carlo Methods to Perform Uncertainty and Sensitivity Analysis on Inverse WaterRock Reactions with NETPATH. United States. https://doi.org/10.2172/1258031
McGraw, David, and Hershey, Ronald L. 2016.
"Application of Monte Carlo Methods to Perform Uncertainty and Sensitivity Analysis on Inverse WaterRock Reactions with NETPATH". United States. https://doi.org/10.2172/1258031. https://www.osti.gov/servlets/purl/1258031.
@article{osti_1258031,
title = {Application of Monte Carlo Methods to Perform Uncertainty and Sensitivity Analysis on Inverse WaterRock Reactions with NETPATH},
author = {McGraw, David and Hershey, Ronald L.},
abstractNote = {Methods were developed to quantify uncertainty and sensitivity for NETPATH inverse waterrock reaction models and to calculate dissolved inorganic carbon, carbon14 groundwater travel times. The NETPATH models calculate upgradient groundwater mixing fractions that produce the downgradient target water chemistry along with amounts of mineral phases that are either precipitated or dissolved. Carbon14 groundwater travel times are calculated based on the upgradient sourcewater fractions, carbonate mineral phase changes, and isotopic fractionation. Custom scripts and statistical code were developed for this study to facilitate modifying input parameters, running the NETPATH simulations, extracting relevant output, postprocessing the results, and producing graphs and summaries. The scripts read userspecified values for each constituent’s coefficient of variation, distribution, sensitivity parameter, maximum dissolution or precipitation amounts, and number of Monte Carlo simulations. Monte Carlo methods for analysis of parametric uncertainty assign a distribution to each uncertain variable, sample from those distributions, and evaluate the ensemble output. The uncertainty in input affected the variability of outputs, namely sourcewater mixing, phase dissolution and precipitation amounts, and carbon14 travel time. Although NETPATH may provide models that satisfy the constraints, it is up to the geochemist to determine whether the results are geochemically reasonable. Two example waterrock reaction models from previous geochemical reports were considered in this study. Sensitivity analysis was also conducted to evaluate the change in output caused by a small change in input, one constituent at a time. Results were standardized to allow for sensitivity comparisons across all inputs, which results in a representative value for each scenario. The approach yielded insight into the uncertainty in waterrock reactions and travel times. For example, there was little variation in sourcewater fraction between the deterministic and Monte Carlo approaches, and therefore, little variation in travel times between approaches. Sensitivity analysis proved very useful for identifying the most important input constraints (dissolvedion concentrations), which can reveal the variables that have the most influence on sourcewater fractions and carbon14 travel times. Once these variables are determined, more focused effort can be applied to determining the proper distribution for each constraint. Second, Monte Carlo results for waterrock reaction modeling showed discrete and nonunique results. The NETPATH models provide the solutions that satisfy the constraints of upgradient and downgradient water chemistry. There can exist multiple, discrete solutions for any scenario and these discrete solutions cause grouping of results. As a result, the variability in output may not easily be represented by a single distribution or a mean and variance and care should be taken in the interpretation and reporting of results.},
doi = {10.2172/1258031},
url = {https://www.osti.gov/biblio/1258031},
journal = {},
number = ,
volume = ,
place = {United States},
year = {2016},
month = {6}
}