Industry: Environmental
Product: ModelRisk
Application: Detection power and sampling-network design for soil and groundwater monitoring

A 9-well monitoring grid had a 32% chance of missing a real contamination hotspot — and the well count alone didn't fix it

A regulator approved a former industrial parcel for redevelopment on the strength of a nine-well groundwater monitoring grid that read clean. The deterministic logic looked airtight: the typical plume on a site like this peaks near 91 ug/L of TCE — well above the 50 ug/L action level — so any plume "would obviously be detected." A Monte Carlo simulation of the plume's uncertain location, size and the measurement error at each well told a different story: that nine-well grid had only a 68% chance of detecting a real exceedance, leaving a 32% probability of missing a genuine hotspot. Reaching the regulator's 95% detection target took 64 wells, not nine — and where the wells went mattered almost as much as how many there were.

Why a peak-concentration point estimate can't answer a detection question

The mistake in the deterministic view is subtle. It compares the plume's peak concentration to the action level and concludes "detectable." But a monitoring well does not sample the peak — it samples one point on a 200 m × 200 m parcel, at whatever distance it happens to sit from a plume whose centre is unknown. What matters is the concentration at the well, after measurement error, not the concentration at the plume core.

We modelled the plume as a radial Gaussian-decay field, C(d) = C_peak · exp(−(d/rho)²), where the peak concentration C_peak is LogNormal (median 90 ug/L, sigma_log 0.85) — skewed, with a long upper tail and occasional near-clean sites — and the decay radius rho is Triangular(18, 35, 70 m): most plumes moderately compact, some diffuse. Each well's reading is the true concentration at its distance multiplied by LogNormal(0, 0.35) analytical-plus-sampling error. A well detects when that reading is at or above 50 ug/L.

Crucially, every well on a given site sees the same plume realisation — one shared peak and one shared decay radius drawn per site. That common factor is what turns detection probability into a distribution across sites rather than a single number. Conditioning on the shared regime makes the point obvious: on strong, wide-plume sites the nine-well grid detects 100% of the time, but on weak, compact-plume sites it detects only 26% of the time — a 74-percentage-point spread driven entirely by the latent plume regime. A model that treated each well as an independent draw would average that away and badly understate how often a whole network comes up empty over a real hotspot.

What the network actually reads

Restricting to sites that genuinely contain an exceedance, the highest reading the nine-well grid returns has a median of 73 ug/L — but a P10 of just 24 ug/L and a P90 of 223 ug/L. On 32% of contaminated sites the entire network reads below the 50 ug/L action level: the plume exists, but no well is close enough to its core to register it. That shaded body to the left of the action line is the missed-detection rate the peak-concentration estimate could not see.

Placement matters as much as count

Doubling the well count is expensive; spending the same wells better is not. At 16 wells, a regular grid achieves 82% detection against 69% for randomly sited wells of the same count — a 14-percentage-point advantage purely from layout. The gap is widest in the mid-range (9–25 wells) where coverage is the binding constraint; once the site is saturated (81+ wells) grid and random converge, because almost any layout finds almost any plume.

Which inputs the miss-rate depends on

Against the nine-well baseline miss probability of 32%, the plume decay radius — how compact the contamination is — moves the miss rate more than any other input (±7.0 points): a tight plume is a small target and is missed far more often than a diffuse one. The number and density of wells is second, well placement third. Measurement error and the exact action-level threshold barely move the number — a useful result, because it says the agency's money is better spent on more and better-placed wells than on a marginally more precise lab method.

What the model changed

• The agency replaced the nine-well grid with a 49-well grid for high-risk parcels, lifting detection power from 68% to 94% and cutting the miss rate from 32% to 6% — short of the 95% target but within the budget envelope, with the residual risk documented rather than assumed away.
• For parcels where 95% detection is mandatory, the design was sized to 64 wells, the smallest grid that crosses the regulatory threshold in simulation.
• Well siting moved from ad-hoc placement to enforced regular-grid layout, banking the ~14-point detection advantage at the well counts that matter, at zero extra drilling cost.
• The defensible monitoring report now states detection power as a probability with its drivers, replacing the indefensible "the plume would be detected" assertion.

ModelRisk Functionality Used

• LogNormal and Triangular inputs for peak concentration, plume decay radius and measurement error — capturing the skewed, heavy-tailed reality of subsurface contamination rather than a single peak value.
• Shared per-site common factor so that all wells on a site sample one correlated plume; the simulation showed detection probability spreading from 26% to 100% across plume regimes, which an independent-well model would have collapsed to a misleadingly stable average.
• Sample-count sweep from 1 to 100 wells, locating 64 wells as the smallest grid meeting the 95% detection target and quantifying the miss rate at every intermediate design.
• Grid-versus-random placement comparison (mean of eight random layouts per count) quantifying a 14-point layout advantage at 16 wells.
• Tornado analysis ranking plume compactness, then well density, then placement as the drivers of the miss rate — pointing the budget at coverage rather than lab precision.

A monitoring network does not detect the contamination it is over; it detects the contamination a well happens to sit close enough to read — and the only honest way to report that is as a probability with a known set of drivers, not as a clean bill of health.