Despite many ambiguities surrounding water quality–evolving regulations, inconsistent industry standards, incomplete performance data for BMPs–a few things have nevertheless seemed certain. You might not know the total maximum daily loads for every water body in your jurisdiction or which BMPs will help you meet them once you do, but until now at least you’ve been fairly certain of what you were measuring. Total suspended solids have long been a standard data point: Many states specify TSS analysis in their water-quality criteria for sediment, and 80% TSS removal is a widespread industry standard for manufactured BMPs. But now it might be changing.

Recent reports from the US Geological Survey assert that TSS data should no longer be used in designing or evaluating the performance of sediment-removal BMPs. The agency says the TSS method, originally devised for wastewater analysis, is spectacularly unsuited to analyzing natural water samples. In particular, sand-size particles–and any larger debris–throw it off. The USGS statement is unambiguous: "Using the TSS analytical method to determine concentrations of suspended material in open channel-flow can result in unacceptably large errors and is fundamentally unreliable…. TSS data can result in errors in load computations of several orders of magnitude."

USGS recommends using suspended-sediment concentration (SSC) data instead for greater accuracy and comparability. The difference, in a nutshell, is this: The TSS method involves measuring the dry weight of sediment from a subsample of an original sample of a water-sediment mixture. Laboratory methods vary, but generally the original sample is mixed with a magnetic stirrer, a small volume–typically 0.1 liter–is removed and filtered, and the contents of the filter are dried and weighed. The SSC method, in contrast, involves measuring the dry weight of all the sediment in the total sample rather than in a subsample.

TSS data will be falsely low, the argument goes, for a sample that contains a significant percentage of heavier, rapidly settling particles such as those frequently found in streams and stormwater pipes. No matter how hard you stir, a subsample isn’t representative of the whole.

USGS has compared the two methods by evaluating thousands of paired SSC and TSS data, noting wide disparities. Especially when more than a quarter or so of the sediment in a sample is made up of sand-size particles, its SSC values tend to be far greater than the TSS values for the paired sample.

Making the switch is no easy matter, as no reliable way exists to convert existing TSS data. If, for reasons of convenience or cost, TSS or some other method (turbidity, optical backscatter instrument data) is used as a surrogate for SSC, USGS cautions, the relationship between SSC and whatever surrogate is used must be defined for each site.

Whenever a fundamental assumption is challenged, nagging doubt creeps in that what we’ve been doing all along might not have been the right thing, that the measurements so diligently gathered–and the work based on those measurements–are intrinsically and unfixably flawed. To comply with regulations, everyone has been running hard in one direction, perhaps to discover midrace that they set out carrying the wrong tools and must go back and start again. If EPA and the industry do ultimately adopt SSC over TSS, the implications for those who are developing and evaluating BMPs, and for water-quality measurements in general, are tremendous. Will some manufacturers start using SSC measurements to show their systems in a more favorable light, while others stick with TSS? As difficult as it now is to compare and document product performance, this scenario would make things worse.

A larger question is whether the BMPs we’re relying on are in fact removing what we thought they were. If TSS measurements consistently show less sediment than SSC measurements for some conditions, are the standards we’ve been using not stringent enough?

USGS doesn’t set policy, and what EPA and the water quality community choose to do with these recommendations is still wide open. The technical merits of one measurement versus the other will continue to be debated, but it points out one thing clearly: the difficulty of establishing and meeting water-quality regulations when the science behind them is still developing.

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