Insights into In Situ Benthic Caging Tests for Ecotoxicity Assessments Targeting Discharging Groundwater Contaminant Plumes

Abstract

While in situ toxicity testing with caged organisms has been used to assess surface water and sediment contamination, no successful application to benthic organisms exposed to highly contaminated groundwater plumes discharging to surface waters has been reported. The objective of this study was to demonstrate and evaluate this application using four sets of tests performed at three previously reported contaminated groundwater sites, which include one river site affected by volatile organic contaminant plumes, and two sites, one pond and one small urban stream, impacted by landfill plumes. The study examined multiple cage designs and orientations and two test organisms: an amphipod (Hyalella azteca) and midge larvae (Chironomus riparius; only one study). Cages were deployed for between 5 and 28 days and assessed for organism survival and growth. At all sites and for some deployment conditions, cages exposed to high contaminant concentrations in the plume footprint had greater mortality compared to those exposed to lower or background concentrations. Organism growth was less clear as a metric of toxicity. Vertically oriented cages typically showed high mortality to plume contaminants, but some were also affected by other non-target groundwater conditions (e.g., low dissolved oxygen, other contaminant sources), while horizontally oriented cages were rarely responsive to either groundwater influence. A hybrid cage design showed much promise in its single study. Useful observations on the test organisms and on potentially problematic site conditions were also made. The informed use of in situ toxicity cages could be an additional beneficial tool for groundwater contaminated site assessments.

Fig. 1

Depictions of the standard and new cage designs and the test organisms used in the various in situ toxicity tests (as summarized in Table 1)

Fig. 2

Survival results for amphipods (Hyalella) from 10 initial organisms/cage over the 7-day toxicity test at the HRM site, with the west location the reference area (a, c, e; (green)) and the east location within the plume footprint (b, d, f (red)) (site diagram in Fig. S5). Each box shows the data interquartile range, with middle line the median, and whiskers at maximum/minimum. The three cage orientations tested were horizontal on top of the sediment (HT; a, b; 4 cages), horizontal and half-buried (HB; c, d; 3 cages) and vertical (V; e, f; 4 cages)

Fig. 3

Average dry weight of surviving amphipods (Hyalella) collected at the end of the HRM 2010 toxicity test, comparing the west (W; outside the plume; green) and east (E; within the plume; red) caging locations (Fig. S5) and three cage orientations: horizontal on top of the sediment (HT; 4 cages), horizontal and half-buried (HB; 3 cages) and vertical (V; 4 cages)

Fig. 4

Results of the HB site in situ caging test for 15 locations (1 horizontal and 1 vertical cage at each) across the study pond (1—east (plume side); 15—west (background side)), showing averaged water quality parameters a specific conductivity (SC), b dissolved oxygen (DO) and c temperature, measured in the cages (with error bars representing 1 standard deviation), along with d minimum survival from 15 initial organisms across the four counting periods (every 7 days), and e average mass of survivors at the end of the study (day 28). Three cages had a malfunctioning screen, so those organism data were omitted

Fig. 5

Results for the DC site (Stretches B and C combined) for 2019, including a saccharin concentrations in shallow discharging groundwater (August), b Hyalella survival from 15 initial organisms following 7-day exposure in vertically oriented cages and c dissolved oxygen (DO) measured in these same cages (day 7); for the 20 cage locations categorized as having low (13), medium (4) and high (3) levels of landfill leachate contamination

Fig. 6

Caging and groundwater sampling results for the DC site Test 2 (Table 1), for four locations on the north (gray bars; landfill-impacted) and south (white bars) sides of the stream, showing 5-day survival for two cages each for a Hyalella and b Chironomus, from ten initial organisms, and concentrations of c the artificial sweetener saccharin, d ammonium-N and e chloride, in co-located discharging groundwater