Total metals and dissolved metals measure different things on a lab report, and the gap between them explains why sediment control alone often fails to move an NPDES or MSGP benchmark exceedance for zinc, copper, or lead.
On a stormwater lab report, dissolved vs total metals answer two different questions: total metals count everything in an unfiltered sample, while dissolved metals count only what passes through a 0.45-micron filter. NPDES and MSGP benchmark monitoring typically digests the total metals fraction, so a facility can show clean sediment control on-site and still fail a benchmark if the dissolved fraction is left untreated.
What Do "Total" and "Dissolved" Actually Mean on a Lab Report?
Total metal concentration equals the dissolved metal concentration plus the particulate, or insoluble, metal concentration. Total metals are measured by digesting an unfiltered sample with strong acids so that any metal bound to a particle is released and counted. Dissolved metals are measured by first passing the sample through a 0.45-micron filter and analyzing only what stays in solution afterward. When an NPDES or MSGP benchmark sample is reported as an exceedance, it is almost always because the total metals number — the sum of both fractions — is over the limit.
That distinction matters because the two fractions behave differently in a treatment system. Particulate-bound metals ride on total suspended solids (TSS) and can, in theory, be filtered or settled out with conventional sediment control. Dissolved metals stay in the water column after solids settle and cannot be removed by physical filtration alone.
Why Sweeping and Berms Only Fix Half the Problem
Facilities operating under the Multi-Sector General Permit (MSGP) — including Sector N for scrap metal and materials recycling and Sector AA for fabricated metal product manufacturing — carry benchmark monitoring requirements for metals precisely because outdoor fabrication and recycling generate metal-bearing residue at nearly every process stage. Galvanized surfaces, cutting and grinding operations, exposed scrap piles, and outdoor storage all contribute both dissolved and particulate-bound metal to runoff before it ever reaches an outfall.
Sweeping and berms address the particulate-bound fraction sitting on TSS. They do nothing for metals already dissolved in the runoff. A facility that only tightens housekeeping and sediment control after a benchmark exceedance for zinc, copper, arsenic, or lead will typically see little movement in its numbers, because the dissolved load was never addressed in the first place. Understanding what causes heavy metals in stormwater by site type is the first step in working backward from the outfall to the sources loading a catchment with every storm.
How Smart Sponge® HM Targets the Dissolved Fraction
Smart Sponge® HM is media selected specifically for dissolved-metal and phosphorus treatment; water chemistry and the target contaminant determine the appropriate configuration. It is designed to address the fraction that sediment-only best management practices (BMPs) cannot reach — the metal already in solution rather than bound to a particle. Because different sites carry different mixes of contaminants, Smart Sponge® HM can be combined with other Smart Sponge® media in a single system, for example pairing metals treatment with sediment and TSS capture where a site's benchmark data shows both fractions are elevated.
This is consistent with Abtech's broader approach across its media family: the media follows the contaminant, and the format follows the site. Before any media is specified, a site assessment reviews contaminant concentrations, typical and peak flows, available space, effluent targets, and maintenance access, because the right response to a metals benchmark exceedance depends on what the site's own water actually contains.
Where Metals Treatment Fits: Catch Basin Retrofits and Trash Capture
For municipal and industrial permit holders, the most practical place to add dissolved-metals treatment is often the structure already in the ground. A catch basin retrofit fits Smart Sponge® media directly inside an existing catch basin sump, converting it from a collection point into a treatment point without excavation, pumps, or power. The Ultra Urban® Filter is the drop-in insert Abtech supplies for this purpose, custom-fabricated to the basin geometry, and it carries a 20-year warranty on the stainless steel insert itself.
Because a standard catch basin sump captures gross solids but allows dissolved and emulsified pollutants to pass through with the water column, pairing a trash and debris capture insert with metals-targeting media addresses both the TSS and the dissolved-metals side of a benchmark sample in the same structure. This same retrofit logic extends to municipal separate storm sewer system (MS4) programs operating under SWPPP requirements, where permit holders need documented pollutant reduction rather than installed hardware alone.
Does the Same Approach Apply to Phosphorus?
Smart Sponge® HM is selected for both dissolved-metal and phosphorus treatment, which matters for municipal wastewater operators managing nutrient limits alongside metals benchmarks. As with metals, phosphorus in a stormwater or wastewater sample can be present in particulate and dissolved forms, and sedimentation or filtration alone typically captures only the particulate share. Facilities evaluating a polishing step for phosphorus should treat it the same way they would treat a metals exceedance: confirm which fraction is driving the number before specifying media.
Frequently Asked Questions
What metals are most commonly tracked in NPDES benchmark monitoring?
NPDES and MSGP benchmark monitoring commonly tracks zinc, copper, lead, aluminum, and iron, with specific sectors such as Sector N (scrap metal and materials recycling) and Sector AA (fabricated metal product manufacturing) carrying explicit benchmark requirements for these metals.
Can source control alone fix a metals benchmark exceedance?
Source control such as sweeping and berms addresses particulate-bound metals sitting on TSS, but it does not remove metals already dissolved in the water column. Facilities that rely on source control alone while ignoring the dissolved fraction typically see limited movement in benchmark numbers.
Do catch basin inserts remove dissolved metals, or just sediment?
A standard catch basin insert without treatment media inside captures gross solids and trash but allows dissolved metals to pass through. Fitting the insert with Smart Sponge® HM media adds a treatment step aimed at the dissolved fraction within the same structure.
Does the same media address phosphorus as well as metals?
Smart Sponge® HM is selected for dissolved-metal and phosphorus treatment together, since water chemistry and the target contaminant determine the appropriate media configuration for a given site.
How do I know which format fits my catchment?
Format depends on the structure already in place and the water conditions at the site — a catch basin favors an Ultra Urban® Filter insert, while a vault, channel, or outfall may call for Smart Pak®, Smart Bags, or an EOP unit. A site assessment reviewing contaminants, flows, space, and maintenance access determines the right combination before anything is specified. You can schedule a site assessment to start that review.