The water supplied to homes in Rapid City, Pennington County, comes from surface water sources managed by a community water system that serves over 72,000 people through more than 23,000 connections. Federal monitoring under the EPA's Unregulated Contaminant Monitoring Rule (UCMR) from 2013 to 2015 provides a detailed look at certain volatile organic compounds (VOCs) detected in this system's water, offering insight into potential concerns for residential water users.
How a Gasoline Release Differs From a Dry-Cleaning Release in What It Leaves in Groundwater
Gasoline-related VOCs and solvents from dry-cleaning facilities represent two major sources of volatile organic compounds in groundwater. Gasoline releases generally contain a complex mixture including benzene, toluene, ethylbenzene, and xylene, whereas dry-cleaning solvents typically involve compounds like perchloroethylene and trichloroethylene. However, the Rapid City system’s surface water source is not influenced by these common groundwater contaminants. The EPA’s monitoring found no detections above reporting levels for several typical gasoline and dry-cleaning solvents such as 1,1-dichloroethane, chloromethane, bromomethane, 1,3-butadiene, and MTBE. This distinction is crucial because it means residential water users here are unlikely to face the types of VOCs commonly associated with those particular contamination events in groundwater.
What a Laboratory's Method Detection Limit Means When a Compound Is 'Not Detected'
When laboratory tests report a VOC as 'not detected,' it means the compound was below the method detection limit, which varies by substance and method. For example, 1,1-dichloroethane was not detected above 0.03 micrograms per liter (µg/L), and MTBE was below 5.0 µg/L in all samples. This does not imply the compound is absent; only that it is below the level the lab can confidently measure. Therefore, absence of detection does not guarantee zero presence—just below detection capability. Residential testing of household water is the only way to verify the presence or concentration of VOCs in that specific supply.
How Seasonal Water-Table Changes Move a Solvent Plume Relative to a Well Screen
Seasonal shifts in water tables can cause any subsurface solvent contamination to move vertically and horizontally, potentially bringing VOCs into or away from the screened zone of a well. However, Rapid City’s supply is surface water, which is generally less susceptible to such plume movements compared to groundwater wells. The variability in VOC detection over time partly reflects this dynamic, but also the nature of the surface water source and distribution system. Homeowners considering treatment should understand that contamination patterns and plume behaviors common in groundwater settings don’t directly apply here.
The Role of Plumbing Material and Age, Separate From the Source Water
VOCs can sometimes enter drinking water downstream from the water source through plumbing materials, especially in older or chemically reactive piping systems. In Rapid City, the system water quality records do not include data on such contributions from household plumbing materials. This means the presence of VOCs in tap water may originate from both the community water supply and private plumbing. Testing at the household level can determine if internal plumbing or storage contributes VOCs, which is important for households deciding on treatment options.
Why Aesthetic Secondary Standards Differ From Health-Based Limits, Applied to This Constituent
Many VOCs do not have federally enforceable maximum contaminant levels (MCLs) because they are not classified as health threats at the measured concentrations, but aesthetic secondary standards may exist for taste or odor. For instance, the detected HCFC-22 concentration reached up to 0.12 micrograms per liter (ppb), which is well below any health-based threshold. Such measurements indicate the presence of a volatile organic compound without signaling a health violation or immediate safety concern. Treatment aimed at primary health protection targets different contaminants and applies different criteria than those for aesthetic concerns.
In conclusion, the EPA monitoring data for Rapid City's public water system in Pennington County shows no health-based violations related to VOCs. Detections at or below laboratory method detection limits and occasional low-level presence of HCFC-22 reflect measured concentrations in this surface water supply rather than water quality issues at individual homes. To establish the VOC profile of your household water specifically, independent laboratory testing of your own tap water is necessary. For households evaluating treatment designed to address VOCs, the Pentair VOCs & Industrial Chemicals Filter offers a documented solution tailored for volatile organic compound removal and can be shipped ready to configure for residential use.
Where this information comes from
Source: EPA SDWIS public water system record. Sampled 2014-04-15. These figures describe the public water system's record, not the plumbing inside any individual building.
On record: EPA UCMR3 monitoring (2013-2015) for Rapid City (SD4600406), SD: 176 results across 7 listed contaminants, 1 detections above the reporting level; no federal MCL for these UCMR-listed VOCs
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