{"id":2010,"date":"2017-03-14T12:00:47","date_gmt":"2017-03-14T19:00:47","guid":{"rendered":"http:\/\/elmontgomery.com\/?p=2010"},"modified":"2022-12-07T14:40:12","modified_gmt":"2022-12-07T21:40:12","slug":"challenging-a-paradigm-what-we-learned-by-comparing-three-common-groundwater-sampling-methods","status":"publish","type":"post","link":"https:\/\/elmontgomery.com\/es\/hydro-notes\/challenging-a-paradigm-what-we-learned-by-comparing-three-common-groundwater-sampling-methods\/","title":{"rendered":"Desafiando un paradigma: lo que aprendimos al comparar tres m\u00e9todos comunes de muestreo de aguas subterr\u00e1neas"},"content":{"rendered":"

As a result of this analysis, M&A obtained regulatory approval to use low-flow sampling methods for long-term monitoring, reducing sampling costs by 75 percent. <\/em><\/p>\n

M&A recently had the opportunity to challenge a long-established practice that hydrologists have applied when collecting groundwater samples. This practice, which has been largely driven by environmental regulators, involves \u201cpurging\u201d three to five casing volumes of water from a well. The idea is to ensure that the sampling captures formation water rather than water that has been resident within the well. Some researchers[1]<\/a> have questioned the efficacy of this approach \u2014 particularly in heterogeneous aquifers with spatially varying concentrations of chemical constituents. However, its simplicity is compelling.<\/p>\n

Unfortunately, the \u201cpurge\u201d method has many practical drawbacks[2]<\/a>. For one thing, in many settings, purge water must be disposed of, which can be costly. In addition, purging can require a significant amount of time, another factor that increases cost. Yet another limitation is the impact of purging on heads due to the large volume of water that must be removed; this unintended consequence can complicate efforts to understand flow directions and rates in a groundwater system. These limitations have led to the development of alternative sampling approaches such as the \u201clow-flow,\u201d minimal-drawdown purge method[3]<\/a>; the HydraSleeveTM<\/sup> method<\/a> (GeoInsight), the Snap Sampler<\/a> (ProHydro, Inc.); and passive-diffusion bag samplers, among others. Each of these methods samples from different zones of the aquifer.<\/p>\n

Comparing the purge, low-flow, and HydraSleeve methods<\/h2>\n

To assess the potential for cost savings associated with long-term monitoring, M&A compared three sampling methods \u2014 purge, low-flow, and HydraSleeve \u2014 at a closed mine site in the western U.S. Samples were collected from 35 wells completed in sedimentary strata ranging from fractured sandstone to mudstone, where the reported hydraulic conductivities ranged from less than 0.001 to over 400 ft\/day and saturated thicknesses ranged from 4 to 170 feet. Not all wells penetrated the aquifer fully. The samples were analyzed for common constituents (TDS, chloride, sulfate, bicarbonate as HCO3<\/sub>, calcium, magnesium, potassium, sodium, carbonate as CO3<\/sub>), and trace metals (uranium, molybdenum, selenium, and arsenic).<\/p>\n

Interestingly enough, the data generally showed good agreement between sampling methods across analytes and wells. Although the results differed between the methods for individual samples, no method produced results with a consistent high or low bias. Relative deviations were found to be heteroscedastic: they were significantly larger among lower-concentration samples than among higher-concentration samples. Additionally, trace metals had larger relative deviations than common constituent and routine parameters. We also found that hydraulic conductivity, saturated screen length, and total well volume did not significantly control comparability between sampling methods, contrary to some regulatory guidelines[4]<\/a>.<\/p>\n

Statistical analysis<\/h2>\n

We conducted a series of sign tests to evaluate the hypothesis that results for the following paired samples would be equal for each analyte (Table 1<\/strong>):<\/p>\n