After a diesel spill on layered alluvium, what reaches a well first? At a 2019 site, a chloride tracer hit MW-4 in about 36 hours via a coarse sand lens (K about 1e-3 m/s), while BTEX lagged for days — lesson learned: track a conservative indicator early to reveal pathways and pick the right remedial focus before chasing the plume.
I’ve seen the same pattern, so I now drop a cheap EC logger in the suspect coarse-lens well and pull chloride every 2–4 hours to pin the “36 hours” front and back-calc velocity before chasing BTEX. Small caveat: if there’s LNAPL perched above that 1e-3 m/s sand, a capillary break can let solute lead while diesel stalls, so I run a low-amplitude slug in the screened interval to confirm connectivity. Curious whether you had continuous EC at MW-4 or only grabs.
Chloride’s the express lane, but I’d also run a short saline pulse and log vertical EC across the screen to pinpoint the entry depth from the lens before BTEX shows. @jennifer_ada66 have you tried a heat-pulse flowmeter during a gentle pump to confirm if that “36 hours” front is dominated by one sub-interval? Caveat: ethanol blends can drag BTEX sooner than you expect.
Road salt masking the early signal drives me nuts, so on layered alluvium like yours — with that K about 1e-3 m/s coarse lens feeding MW-4 — I’ll run bromide instead of chloride and read it with a cheap Br- ISE for a clean early breakthrough. @jennifer_ada66, have you tried bromide on your saline pulses to keep the curve crisp? Just watch for minor anion exchange if nearby clays are present; in the coarse sand it’s effectively conservative.
One tweak I’d try at MW-4: a 1–2 hr low-flow step test while logging EC and drawdown; the first sharp EC break tags the high-K sand connection and lets you back-calc apparent velocity so you aren’t waiting a day-and-a-half for BTEX to show, . Pairs nicely with @jennifer_ada66’s entry-depth logging, and it’s cheap. Was MW-4 a short screen or open over much of that interval?