Question 2 of 8: Leachate Sampling Program for a Closed Hazardous Waste Landfill

Question A-2: Leachate Sampling Program for a Closed Hazardous Waste Landfill (20 marks)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

A suspected liner failure turns this from a routine post-closure obligation into a legal/regulatory investigation, so the sampling program has to be defensible in a chain-of-custody sense as well as scientifically sound.

Sample locations. Collect from (1) the leachate collection system (LCS) sumps/manholes at the low points of the collection piping — this is the “source” sample characterizing what the liner was designed to contain; (2) any leak-detection-layer sump beneath the primary liner, if present, since flow there directly proves a breach; (3) a ring of groundwater monitoring wells already required around a hazardous waste landfill under its post-closure permit, split into at least one upgradient (background) well and several downgradient wells along the inferred flow path; and (4) surface water/seep points if any daylighting of leachate at the toe of the landfill is observed. The upgradient well is essential — without it there is no way to prove a downgradient exceedance came from this landfill rather than another regional source.

Frequency. Under a typical post-closure monitoring permit, LCS leachate and the groundwater ring are sampled quarterly during the first 1–2 years of an active investigation (to capture seasonal water-table and generation-rate variability), stepping down to semi-annual once a stable trend is established; if the initial round confirms an exceedance, sampling should increase to monthly at the affected wells until the plume is delineated and a response plan is in place. A single “grab and done” sample cannot support a defensible finding either way.

Parameters. Field parameters at every round (pH, conductivity, temperature, dissolved oxygen, ORP — measured in a flow-through cell, not after exposure to air) plus a full leachate characterization suite: general chemistry (COD, BOD5, TDS, chloride, sulfate, alkalinity), nutrients (ammonia-N, TKN), metals (full RCRA 8 at minimum, since a hazardous waste landfill’s waste stream is unknown to the sampler), and volatile/semi-volatile organics (VOCs/SVOCs) by EPA 8260/8270, since chlorinated solvents and other VOCs are the most mobile and toxicologically significant leachate constituents. The specific waste-acceptance history of the landfill (if documented) should narrow this list further — e.g., add cyanide or PCBs if those wastes are known to have been received.

Collection. Dedicated, decontaminated low-flow bladder pumps for monitoring wells (minimizes volatilization/degassing of VOCs and cross-contamination between wells); grab sampling directly from LCS sumps with a disposable bailer; purge each well to stable field parameters (not a fixed number of well volumes) before sampling; VOC vials filled with zero headspace; full chain-of-custody documentation (sampler, time, location, preservation) started at the moment of collection, not at the lab.

Storage and shipping. Samples are placed on ice (4 ± 2 °C) immediately in the field and kept iced through transport; metals samples preserved with nitric acid to pH < 2, VOC vials with no headspace and no preservative disturbance; each analyte has a regulatory holding time (VOCs 14 days to analysis, metals 6 months, BOD 48 h) that governs shipping urgency — VOC and BOD samples are shipped same-day/overnight to an accredited (CALA/ISO 17025) laboratory with a signed chain-of-custody form accompanying the cooler, and a trip blank/field blank/duplicate accompany every sampling event for QA/QC.