Time vs. Money: A Quantitative Evaluation of Monitoring Frequency vs. Monitoring Duration

Authors: T. McHugh, P. Kulkarni, C. Newell
Published: February 2016 in Ground Water.

Abstract

The National Research Council has estimated that over 126,000 contaminated groundwater sites are unlikely to achieve low
ug/L clean-up goals in the foreseeable future. At these sites, cost-effective, long-term monitoring schemes are needed in order to
understand the long-term changes in contaminant concentrations. Current monitoring optimization schemes rely on site-specific
evaluations to optimize groundwater monitoring frequency. However, when using linear regression to estimate the long-term
zero-order or first-order contaminant attenuation rate, the effect of monitoring frequency and monitoring duration on the accuracy
and confidence for the estimated attenuation rate is not site-specific. For a fixed number of monitoring events, doubling the
time between monitoring events (e.g., changing from quarterly monitoring to semi-annual monitoring) will double the accuracy of
estimated attenuation rate. For a fixed monitoring frequency (e.g., semi-annual monitoring), increasing the number of monitoring
events by 60% will double the accuracy of the estimated attenuation rate. Combining these two factors, doubling the time between
monitoring events (e.g., quarterly monitoring to semi-annual monitoring) while decreasing the total number of monitoring events
by 38% will result in no change in the accuracy of the estimated attenuation rate. However, the time required to collect this dataset
will increase by 25%. Understanding that the trade-off between monitoring frequency and monitoring duration is not site-specific
should simplify the process of optimizing groundwater monitoring frequency at contaminated groundwater sites.