The Arsenic Exceedance at MW-7
At a closed municipal landfill, semiannual groundwater monitoring has returned arsenic at 0.014 mg/L in monitoring well MW-7, exceeding the 0.010 mg/L MCL. Turbidity is 210 NTU, dissolved oxygen 0.8 mg/L, pH 6.4, and specific conductance 640 µS/cm. The sample was collected by low-flow purge, but turbidity never stabilized; the lab analyzed total arsenic. The field duplicate is 0.013 mg/L; the equipment blank is <0.001 mg/L. The previous two events at MW-7 were 0.006 and 0.008 mg/L. No other wells exceed. The landfill permit requires notification within 14 days after receipt of validated data. The lab delivered the report 3 days ago. The site operator says the arsenic is 'naturally occurring' and wants to wait for the next semiannual event.
1. Is the 0.014 mg/L arsenic result valid and representative? Explain the sampling artifacts and data-quality checks you would evaluate before deciding.
Read the worked answer
The result is analytically valid enough to trigger review, but it is not necessarily representative of formation water. A turbidity of 210 NTU indicates the sample contains suspended sediment; arsenic readily adsorbs to iron and manganese oxides and clay particles. Even though low-flow purging was attempted, the purge did not stabilize, so the sample may overstate dissolved arsenic. Because the lab analyzed total arsenic, the MCL comparison is appropriate for total, but the result may reflect colloids rather than groundwater. I would check the calibration and continuing calibration blanks, laboratory control sample recovery, matrix spike recovery, and the field duplicate agreement; the duplicate at 0.013 mg/L confirms precision but not representativeness. The equipment blank is clean, so equipment contamination is unlikely. I would also review the purge log to see whether drawdown or a damaged well screen caused the turbidity. The previous results of 0.006 and 0.008 mg/L suggest a possible transient artifact, but that does not justify ignoring the exceedance. The correct conclusion is that the data are usable for notification but technically qualified as possibly turbidity-influenced.
2. What actions should you take now, including notification, resampling, and evaluation of the 'naturally occurring' claim?
Read the worked answer
I would notify the regulatory agency within the permit's 14-day window, because the result has been validated and the deadline runs from receipt of validated data; waiting for the next semiannual event would violate the permit. The notification should state the result, the elevated turbidity, the duplicate, and the planned resampling. I would resample MW-7 as soon as practicable using improved low-flow procedures: monitor drawdown, reduce the pumping rate, and do not collect until turbidity is stable and as low as practicable, ideally below 5 NTU. I would collect both filtered and unfiltered metals samples, with field duplicates and an equipment blank, and analyze for arsenic plus iron, manganese, and sulfate to evaluate geochemical conditions. I would also sample nearby wells to determine whether the exceedance is isolated. The 'naturally occurring' claim requires evidence: site geology, background wells, arsenic-iron correlations, and historical data. It should not be accepted based on one turbid sample. If resampling confirms a sustained exceedance, the landfill should evaluate whether the monitoring network or statistical analysis needs revision under the permit.
3. How would you document this event in the monitoring report so that the data are transparent and defensible?
Read the worked answer
The report should present the arsenic result exactly as reported, flag it as an exceedance of the MCL, and include a data-qualification discussion. I would include the field parameters, purge log, turbidity readings, duplicate result, equipment blank, and laboratory QC summary in an appendix. The narrative should explain that the sample was collected by low-flow methods but turbidity did not stabilize, that total arsenic was analyzed, and that the result may include particle-associated arsenic. I would avoid conclusory language such as 'naturally occurring' unless supported by the resampling and geochemical data. The report should state the notification date and agency contact, describe the resampling plan and schedule, and identify any data gaps. If the resample is below the MCL, the report should not simply delete the original result; it should retain both results and explain the difference. If the resample confirms the exceedance, the report should recommend a permit-required assessment. A clear timeline and chain of custody for the resample should be included so the record shows a consistent, non-defensive response.