In TapWaterSafety’s data, water systems that draw from groundwater (wells) grade better on average than systems that draw from rivers, lakes and reservoirs. Groundwater systems average a score of 72.1 versus 67.2 for surface water systems, and 14% earn an A compared with 6%. But the two source types have different weak spots. Surface water systems are far more likely to show high disinfection byproducts. Groundwater systems are more likely to show naturally occurring contaminants such as arsenic, chromium-6 and radium.
Neither source is automatically safe or unsafe. What matters is how a specific utility treats its water and what shows up in its monitoring.
The data we compared
TapWaterSafety’s dataset covers 1,000 US public water systems, each serving roughly 50,000 people or more. Each system lists its primary source as reported to EPA. For this comparison we grouped them as follows:
- Surface water (764 systems): 474 that treat their own surface water plus 290 that buy treated surface water from another utility.
- Groundwater (227 systems): 224 that use their own wells plus 3 that buy groundwater.
- Groundwater under the direct influence of surface water (9 systems): wells that behave like surface water and must be treated like it. We left these out of the comparison because the group is too small.
EPA compliance data is from EPA ECHO compliance records, checked October 7, 2026, covering the last 12 quarters (three years).
Grades by source type
| Surface water | Groundwater | |
|---|---|---|
| Systems | 764 | 227 |
| Average score | 67.2 | 72.1 |
| Median score | 68 | 73 |
| A | 6% | 14% |
| B | 23% | 33% |
| C | 33% | 28% |
| D | 23% | 19% |
| F | 15% | 7% |
Groundwater systems are about twice as likely to earn an A and about half as likely to get an F. Part of the reason is built into the grading. Source water vulnerability is 10% of the score, and the methodology weighs source type there. The larger part comes from contaminant results, covered below.
EPA compliance records by source type
| Last 12 quarters | Surface water | Groundwater |
|---|---|---|
| Any quarter with a violation | 39.7% | 54.6% |
| Average quarters with a violation | 1.86 | 3.98 |
| At least one health-based violation entry (MCL, MRDL or treatment technique) | 9.2% (70) | 7.9% (18) |
| Listed by EPA as a serious violator | 1.2% (9) | 11.0% (25) |
| Over the lead action level | 21 systems | 1 system |
At first glance groundwater systems look worse on compliance. The detail tells a different story. All 25 groundwater serious violators are in Florida, and Florida accounts for 76 of the 227 groundwater systems. Of Florida’s groundwater systems, 86.8% had at least one violation quarter.
Outside Florida, the two source types look almost identical: 38.4% of groundwater systems and 39.0% of surface water systems had at least one violation quarter, and about 9% of each had a health-based entry. This data does not explain why Florida’s numbers are higher, so we would not draw conclusions about groundwater as a whole from it.
Most violations in both groups are monitoring and reporting violations, meaning a required test, report or notice was late or missing. That is different from finding a contaminant above a legal limit. The most common entries for groundwater systems were under the Revised Total Coliform Rule (monitoring) and the Stage 2 Disinfection Byproducts Rule (monitoring). For surface water systems, Lead and Copper Rule monitoring and the Consumer Confidence Report rule led the list. See what an EPA serious violator is for more on that label.
On lead, 21 surface water systems and 1 groundwater system were over the action level. Lead usually comes from service lines and household plumbing, not the source, and is driven by how corrosive the water is. Read how to find out if you have a lead pipe.
Top concerns by source type
Share of systems listing each item among their top concerns:
| Concern | Surface water | Groundwater |
|---|---|---|
| Haloacetic acids (HAA9) above health guideline | 86% | 62% |
| Total trihalomethanes above health guideline | 59% | 50% |
| HAA5 above health guideline | 56% | 34% |
| Any PFAS detected | 39% | 40% |
| PFHxS above health guideline | 20% | 32% |
| Arsenic above health guideline | 19% | 29% |
| Chromium-6 above health guideline | 3% | 13% |
| Radium above health guideline | under 1% | 6% |
| Nitrate above health guideline | 0% | 4% |
“Health guideline” here means EWG’s health guideline, which is usually much stricter than EPA’s legal limit. Being on this list does not mean a system broke a federal rule.
Why surface water tends to have more disinfection byproducts
Rivers and lakes are open to the environment and to runoff. Because of that, federal rules require nearly all systems using surface water, or groundwater under the direct influence of surface water, to filter and disinfect it. Those rules target germs such as Giardia, Cryptosporidium, viruses and bacteria.
Disinfection is essential, but it has a side effect. Chlorine and other disinfectants react with natural material in the water to form disinfection byproducts, including trihalomethanes and haloacetic acids. EPA notes that byproduct levels depend on source water type, temperature, the amount of natural organic matter and the disinfectant used. Surface water usually carries more organic matter, which fits what our data shows: HAA9 and HAA5 concerns are much more common for surface water systems.
EPA’s legal limits are 80 ppb for total trihalomethanes and 60 ppb for HAA5. See our guide to TTHMs and HAA5.
Surface water can also be affected by harmful algal blooms. In 2015, EPA issued non-enforceable health advisories for two algal toxins, microcystins and cylindrospermopsin, with lower values for bottle-fed infants and young children.
Why groundwater tends to have more natural contaminants
Groundwater is filtered as it moves through soil and rock, which removes many germs and particles. But as water sits in contact with rock, it can dissolve minerals, some of them harmful.
The U.S. Geological Survey sampled untreated water from 932 public-supply wells in parts of 41 states (1993 to 2007). It found that 22% of samples had at least one contaminant above a human-health benchmark. Of the 10 contaminants that most often exceeded benchmarks, seven occur naturally: radon, radium, gross alpha radioactivity, arsenic, manganese, strontium and boron. The man-made ones were nitrate, the pesticide dieldrin, and the solvent PCE. These were untreated samples, so they show what treatment has to deal with, not what reaches the tap.
That matches the pattern in our data: arsenic, chromium-6 and radium concerns are all more common for groundwater systems. Learn more in our guides to arsenic, chromium-6, radium and uranium and nitrate.
Groundwater is not germ-proof. EPA’s Ground Water Rule, published in 2006, exists because wells can be contaminated by fecal material, for example from a nearby leaking septic system. It requires sanitary surveys, follow-up testing after positive coliform results, and corrective action for systems at risk.
PFAS: a concern for both
PFAS showed up at nearly the same rate for both source types (39% of surface water systems and 40% of groundwater systems). PFHxS was more often above the health guideline in groundwater systems. EPA’s April 2024 rule set limits of 4.0 ppt for PFOA and PFOS and 10 ppt for PFHxS, PFNA and GenX. In May 2026, EPA proposed rescinding the PFHxS, PFNA, GenX and mixture limits and allowing PFOA and PFOS compliance until 2031. As of October 2026 these are still proposals, not final rules. See our PFAS rule explainer.
What this means for you
- Know your source. Your utility’s annual Consumer Confidence Report names it. Our guide on how to read your water quality report shows where to look.
- Match concerns to source type. On surface water, look at the disinfection byproduct rows. On groundwater, look at arsenic, radium, chromium-6 and nitrate.
- Private wells are different. Everything above is about public systems. Private wells are not regulated under the federal Safe Drinking Water Act, and owners are responsible for testing. See our well water testing guide.
Records can change. A utility may have fixed an issue since our data was pulled, so check the EPA report linked on your utility’s page and its latest Consumer Confidence Report. To see your own system’s source, grade and record, search by ZIP code.
Sources
- TapWaterSafety methodology
- EPA ECHO: Enforcement and Compliance History Online
- USGS: Quality of Source Water from Public-Supply Wells in the United States, 1993-2007
- EPA: Surface Water Treatment Rules
- EPA: Ground Water Rule
- EPA: How does EPA regulate disinfection byproducts?
- EPA: Drinking water health advisories for cyanotoxins
- EPA: PFAS National Primary Drinking Water Regulation
- EPA: National Primary Drinking Water Regulations