You might notice that local water quality reports mention PFAS compounds at levels expressed with four decimal places in nanograms per litre (ng/L), also known as parts per trillion (ppt). For example, values like 0.004 ng/L are typical reporting limits for individual PFAS chemicals in Rapid City's municipal water, which draws from surface water sources in Pennington County. Understanding these small numbers is key to making an informed treatment choice.
Why PFOA and PFOS Are Measured with Four Decimal Places in Nanograms per Litre
PFAS—per- and polyfluoroalkyl substances—are detected using highly sensitive instruments. Measurements in nanograms per litre correspond to parts per trillion, reflecting the extremely low concentrations of these fluorinated "forever chemicals" in water. Four decimal places make it possible to distinguish between no detection and trace detections above the reporting level, which is essential for monitoring trends and regulatory compliance.
Why Granular Carbon Beds and Ion-Exchange Resins Are Documented Differently for PFAS
Treatment technologies vary in how they reduce PFAS from water. Granular activated carbon (GAC) beds adsorb some PFAS compounds onto their surfaces, while ion-exchange resins swap contaminant ions for harmless ones. Because these processes differ fundamentally, records documenting their performance list them separately. This distinction matters when comparing equipment classes in relation to your household's water quality and demand.
A Worked Example: Reading the Record's Median and Range as a Homeowner Would
The 2023–2025 EPA unregulated contaminant monitoring data for Rapid City's water system showed two detections of PFHxS out of 20 samples, with detected concentrations ranging from 0.0049 to 0.0056 micrograms per litre (µg/L). Converted to nanograms per litre, this corresponds to 4.9 to 5.6 ng/L, all well below the federal maximum contaminant level (MCL) of 10 ng/L effective in 2024. No PFOA or PFOS were detected above their reporting limits, and no results exceeded MCLs. This means the water system’s record indicates very low trace levels—not a safety concern or legal exceedance.
How Sample Depth, Well Depth, or Source Type Changes What a Reader Should Expect
Rapid City’s water comes from surface sources rather than wells. Surface water typically has different PFAS profiles compared to groundwater, influenced by land use, runoff, and atmospheric deposition. Homes on private wells may experience distinct PFAS concentrations that can only be established by testing their specific water supply. Hence, individual water tests remain essential to understand the PFAS situation at a given home.
How to Compare This Area’s Record with a Neighboring County’s Without Overreading It
Comparing data between counties requires attention to monitoring scope, sample sizes, and source types. Pennington County’s public water system shows no detections of PFOA or PFOS above reporting levels and very low PFHxS detections, all below federal MCLs. A neighboring county may have different detections or source types, but this does not imply one area's water is unsafe or another’s is problematic. Each system’s data reflect systemic measurements, not individual home water quality.
For homeowners balancing water demand with removal of trace PFAS compounds, the Fleck PFAS Forever Chemicals Filter ships ready to configure. It uses proven adsorption and ion-exchange technologies designed to address the detection levels typical of Pennington County’s public water system records, providing a tailored solution informed by your household’s measured demand.
Where this information comes from
Source: EPA SDWIS public water system record. Sampled 2025-09-03. These figures describe the public water system's record, not the plumbing inside any individual building.
On record: EPA UCMR5 monitoring (2023-2025) for RAPID CITY (SD4600406), SD: 200 results across 10 listed contaminants, 2 detections above the reporting level; compared with the applicable federal MCL, none above
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