Storm response and data continuity for monitoring programs

Storm frequency and site data risk

About 100 of the nation's most contaminated toxic waste sites sit in flood-prone areas, according to the EPA's Office of Inspector General. Roughly 3 million Americans live within a mile of one of these sites.

The pattern isn't slowing down. In 2025, flash flooding triggered more warnings than any year on record and touched communities in 99% of U.S. counties. Tropical Storm Arthur brought record rainfall to the Gulf Coast in June 2026. Extreme flooding hit Columbus, Ohio two months later. By late August, flash flood warnings stretched across the entire Northeast.

For a long-term monitoring program, that pattern creates a quiet risk. An unflagged storm event can skew trend data and compromise years of recorded monitoring, not because the program was wrong, but because a storm-affected sampling round reads the same as any other round unless someone documents it as different. The response window is short: hours, not days.

Download the field checklist for the full response sequence, step by step. Keep it in the sampling kit before the next event. [Insert checklist link]

Variances in contaminant mobility

Mobility behavior varies by site geology, contaminant phase and storm intensity, so treat the table below as a starting reference for response planning, not a predictive model.

PFAS note: A 2025 peer-reviewed study (Saleh et al., Stevens Institute / Water) found PFAS persisting in urban stormwater runoff following rainfall events, with short-chain compounds showing the highest surface water mobility. Sites with a firefighting training, industrial laundry or landfill history warrant priority attention after storm events.

Turbidity thresholds and sampling sequence after a storm

One number does a lot of the work here. Flag turbidity readings above 10 NTU, and treat anything above 50 NTU with caution and document it accordingly. A storm-disturbed well can produce a reading that looks like a real change when it isn't, and the number alone won't tell you which.

See the full sequence in the field checklist.
It covers timing, thresholds and COC flagging in one printable reference.

Sequence follows the same logic. Surface water and sediment stabilize fastest after a storm, so collect those first, within 24–72 hours where conditions allow. Don't skip sediment. Storm-transported metals, hydrocarbons and PFAS end up there most often. Groundwater takes longer, since a sample collected too early reflects the storm still moving through the well rather than the plume itself.

Flag storm-event samples on the COC at submission, noting turbidity, water level status and any protocol deviation, so a later reviewer can tell the difference too. Holding times don't move for storm-event samples, whatever else the schedule looks like.

Telling a storm-driven exceedance from real plume migration

Most anomalies after a storm turn out to be the storm, not the plume. These indicators point to transient storm conditions:

• Elevated turbidity at time of collection (documented on COC)
• Water levels significantly above seasonal baseline at time of sampling
• Anomalous detections limited to the storm-event sampling round, with return to baseline in the subsequent round
• Detections correlated spatially with surface water inundation extent rather than established plume geometry
• Metals or PFAS detections in wells with documented turbidity above 50 NTU

A smaller set of signals is worth escalating. These indicators warrant further evaluation:

• New detections in historically clean perimeter wells not correlated with turbidity
• Dissolved-phase concentrations significantly above pre-storm baseline in multiple downgradient wells
• LNAPL presence in wells with no prior free-product history
• Detections at locations outside the historic plume boundary that persist into the next sampling round

Regulators reviewing post-storm data look for the same signals. The COC notation, field log and pre-storm baseline support that conversation, whether it happens before or after results are reported.

ALS maintains storm-surge capacity across regional laboratories, and if TAT requirements or sample volumes climb during a storm-response event, your account manager or the nearest ALS laboratory can help directly.

Resources

Download the ALS Storm Event Response Checklist.
This field-ready reference card covers the full post-storm response sequence, COC flagging and regulatory notification triggers. [Insert checklist link]

Talk to an ALS technical resource for method capability, detection limits, storm-surge capacity or COC questions specific to your site and program. [Insert contact link] or contact your ALS account manager directly.