Turbid Well, Two Arsenic Results, Thirty-Day Deadline
At a former smelter site, a monitoring well (MW-7) is screened in a sandy aquifer 7–9 m below grade, downgradient of a slag pile. The QAPP requires low-flow purging at ≤0.3 L/min until pH, specific conductance, and turbidity stabilize, then field filtration at 0.45 µm and nitric acid preservation for dissolved metals. During the July event, MW-7 ran turbid (55 NTU) after 45 minutes and never stabilized below 10 NTU, so the sampler collected it anyway. Two bottles were filled: one filtered in the field, one left unfiltered and filtered at the laboratory 72 hours later. Results: field-filtered dissolved arsenic 9 µg/L, lab-filtered 47 µg/L; field-filtered lead 6 µg/L. The field duplicate for lead returned 13 µg/L and 6 µg/L, and the laboratory reported a matrix spike recovery of 62% for arsenic. Total suspended solids at MW-7 were 210 mg/L with 8 mg/L iron. The client wants to submit all results to the regulator to meet a 30-day reporting deadline.
1. Which arsenic result represents dissolved arsenic, and what explains the 9 versus 47 µg/L difference and the 62% spike recovery?
Read the worked answer
The 9 µg/L field-filtered result is the defensible dissolved value. Filtration must precede acidification, in the field, at 0.45 µm.
The 47 µg/L almost certainly came from an aliquot that stood unfiltered for 72 hours and was acid-preserved before filtration. Acid dissolves the iron-oxyhydroxide and clay colloids that carry sorbed arsenic, releasing that sorbed arsenic into what the laboratory reports as the dissolved fraction. The supporting evidence is direct: 55 NTU, TSS of 210 mg/L, and 8 mg/L iron - exactly the colloidal load that produces this bias. The measurement is an artefact of sample handling, not a property of the groundwater.
But the field-filtered value is also qualified:
- The well never stabilised. The sample fails the QAPP representativeness criterion, and a turbid sample can still carry colloidal arsenic if the filter loads or tears. Report it as estimated, with the turbidity documented.
- A matrix spike recovery of 62% falls outside the usual 75–125% acceptance window, indicating suppression or a spectral interference in the arsenic determination. Every arsenic result in that batch carries the same doubt and should be flagged, diluted, and reanalysed, with standard addition if the matrix is the cause.
Net position: 9 µg/L is usable as a qualified estimate of dissolved arsenic; 47 µg/L should be rejected as a handling artefact and must not be reported as a compliance value. Neither supports a clean compliance determination at MW-7.
2. Calculate and interpret the duplicate lead pair. Should the laboratory or the sampling method be blamed?
Read the worked answer
RPD = |13 − 6| / [(13 + 6)/2] × 100 = 36.8%.
A typical QAPP criterion for aqueous field duplicates is ±20% (sometimes ±25% for metals), so on its face the pair fails. Apply the near-blank rule first. Precision criteria are normally not applied when results fall below about five times the reporting limit. If the lead reporting limit is 5 µg/L, the two values sit at roughly 1.2 and 2.6 times the limit, where counting statistics, filter loading, and incidental contamination dominate the variance.
What the pair actually shows: one aliquot picked up particulate lead - the same turbidity problem that inflated the arsenic. Above five times the reporting limit the pair would test the method; here it flags the well and the sampling conditions.
Actions:
- Treat both values as estimated and do not use them to compute a UCL or to compare with a standard.
- Recognise that a field duplicate from a turbid well is a poor test of laboratory precision. The correct design is a laboratory duplicate from the same bottle (tests the lab) plus a field duplicate at a low-turbidity well (tests the field procedure).
- Add iron, manganese, and aluminium to the analyte list at this well as colloid indicators, so future turbidity effects can be diagnosed rather than guessed at.
3. Recommend how to meet the 30-day reporting deadline while keeping the data defensible.
Read the worked answer
Submit on time, with a case narrative. Do not withhold data and do not present unusable data as compliant.
In the report: the field-filtered results with appropriate qualifiers; the lab-filtered result reported but flagged as rejected, with the reason (unfiltered hold, acid dissolution of colloids); the matrix spike failure and the affected batch; the duplicate pair with the RPD noted and the near-limit caveat. A regulator will accept qualified data with an explanation far more readily than an unexplained gap or an unqualified false exceedance.
Within 72 hours: reanalyse the preserved aliquot with dilution and standard addition to resolve the 62% recovery; determine whether the whole analytical batch is affected - if it is, every metal in that batch carries the same qualifier.
Resampling plan: redevelop MW-7 with a dedicated low-flow pump; if turbidity will not fall below 10 NTU, sample with in-line filtration, record the stabilisation data, and document the deviation. Add a 0.1 µm ultrafiltration sensitivity check to demonstrate whether the residual is truly dissolved or colloidal. Resample within 14 days and submit a supplement referencing the original report.
Root cause to fix in the QAPP: add a stop-work rule. If turbidity does not stabilise, the sampler must document it and proceed with in-line filtration rather than filling bottles anyway. Also require field filtration immediately after collection with preservation recorded on the chain-of-custody - this episode would not have occurred under that rule.