Mill effluent fish toxicity, toxicity reduction and aquatic effects studies

Routine, semi-annual, 96-hour static acute lethality tests have been carried out on mill effluent for many years. Rainbow trout fry (0.5 - 10 g) are the standard reference fish, selected because they are generally available throughout Canada, their responses to most metals are fairly well known, and the water which they inhabit are representative of good water quality in Canada.

In November 1993, the Key Lake mill effluent failed one of these routine tests (>50%

mortality). This prompted an investigation, which ultimately rectified the acute toxicity problem. A Toxic Identification Evaluation (TIE) study was carried out. This study involves a systematic fractionation of the effluent mixture in order to characterize, isolate, and identify the toxic constituent(s). Isodecanol, which is added to the kerosene/tertiary amine solvent to assist in organic/aqueous phase separation in solvent extraction, was identified as the substance responsible for rendering the effluent marginally toxic to trout, despite its relative

insolubility in water. The problem was eventually traced to a change in ore mineralization in the deeper reaches of the Deilmann deposit (basement rock hosted vs. sandstone hosted mineralization). This ore change increased silica and calcium inputs to solvent extraction, which in turn, led to higher raffinate organic losses. The processing problem was resolved in 1994, through better silica/calcium control in solvent extraction feed, lower isodecanol levels in solvent, reduced raffinate organic loss and organic removal from effluent during treatment by aeration and heat.

The investigation also showed that for the toxicity problems encountered at Key Lake, there was a defined relationship between 96-hour rainbow trout tests and a 15-minute microbioassay test (Microtox), which measures the inhibition of light production from a strain of luminescent bacteria as a sign of toxic response. This quick biological test is now used on all pond releases to control the effluent acute toxicity.

Follow-up investigations were carried out, and formed one of the issues tracked in this CRP. It was decided to assess the environmental effect of the 1994 problem on the near-field mill effluent drainage system. A study was designed with the following objectives:

— Review known literature on the ecological fate and effects of isodecanol in aquatic systems;

— Verify the conclusions of this literature-based evaluation using appropriate field measurements; and

— Document any shift in the aquatic biological resources of the near-field drainage. This aspect had been extensively documented in 1993 as baseline work for the proposed McArthur River development.

The literature review concluded that the levels of isodecanol in the effluent discharge in 1994 probably had little or no measurable effect on the receiving aquatic system. It was felt that isodecanol would likely be lost through co-precipitation, vaporization, non-specific adsorption and bio-degradation. The bio-accumulation potential was considered small, perhaps non-existent. Field water-quality measurements failed to find differences in isodecanol concentrations between reference and effluent receiving waters (to a 0.2 mg/L detection level).

Water quality and limnological data were similar to previous operational investigations, being consistent with known effluent influences. Macroinvertebrate community structure and composition changes were evident between 1993 and 1995; however, these differences were judged minor and of little significance to the overall functioning of the aquatic ecosystem.

Fish studies were purposefully restricted in the study due to a desire to use non-destructive sampling methods. Concerns had been raised that the limited fish population needed time to recover from extensive fish sampling programmes conducted in 1993 and 1994, given the relatively low productivity of the small, shallow, cold, nutrient poor, sandy substrate lakes found in the area. Spawning site assessments were generally inconclusive, and lake fish population presence was only determined by indirect evidence (fish sounders). Nevertheless, fish avoidance of the area due to increased pollutant presence in the receiving water was not evident.

Attempts to conduct sediment toxicity tests using non-standardized procedures proved somewhat problematic. Benthic macroinvertebrate testing showed no discernible impact on sediment dwelling organisms. Fathead minnow larvae tests initially showed a difference from controls, believed due to pH suppression during the test. Follow-up tests had control problems

due to turbidity. Therefore, minnow tests were judged inconclusive. Rainbow trout tests showed no transfer of contaminants at toxic concentrations to the water column.

The 1995 programme would likely have brought closure to the issue; however, in the last field investigation in September 1995, a relatively high incidence of skin lesions was found in fish populations within the drainage, particularly in near-field bottom-feeding white sucker populations. This brought a new dimension to the follow-up study.

Follow-up investigations in late 1995 and 1996 focused on fish disease epidemiology.

Pathological examinations defined the problem as an outbreak of a haemorrhagic ulcerative skin disease, associated with the presence of elevated levels of certain bacterial strains.

Literature reviews showed that the type of problems encountered at Key Lake have been seen elsewhere, in relation to deteriorating water quality, high stocking densities in farmed fish, reduced dissolved oxygen levels, high organic loading, parasitic infections, domestic sewage, other pollutants and prior disease outbreaks (i.e., primary pathogen vs. secondary or opportunistic pathogen). A variety of hypotheses were formed for the site-specific Key Lake outbreak of disease:

— Presence of treated domestic sewage effluent in the same lakes which receive the mill effluent;

— Elevated water temperatures during the summer of 1995;

— Outbreak of a particularly virulent strain of previously benign bacteria in the drainage;

— General immuno-suppression or chronic stress in resident fish populations from some as yet unidentified substance or condition, leading to opportunistic bacterial infections be it effluent related or natural in origin (such as spawning stress); and

— An artifact of generally low pH within the focal lakes. Receiving waters have very low buffering capacity, and ammonia control dictates lower-end pH discharges to mediate ammonia toxicity (average pH of 6.31 in 1995, with total ammonia levels averaging 12.8 mg/L).

The focal point of the disease was located in near-field lakes, which received mill effluent and domestic sewage effluents, and it was primarily associated with white suckers. Disease incidence and severity further downstream appeared to be significantly lower. Bacterial densities in water were similar in effluent-exposed and reference lakes, suggesting water-borne bacteria densities were not a primary factor. Host susceptibility differences between the reference and influenced water bodies, for an as-yet undetermined causal agent, seemed likely.

In 1997, it was concluded that the initial disease outbreak likely occurred sometime between mid-1994 and mid-1995. The disease has been identified as a bacterial haemorrhagic septicemia (red-spot) with evidence supporting a specific diagnosis of Motile Aeromonad Septicemia (MAS). The disease progression is different from other reported MAS outbreaks in that death of the fish was not the disease outcome. Cold water temperatures may have provided the fish with a temporary respite from the disease, leading to a chronic disease with late stage lesions, showing evidence of cycles of healing and reactivation. It is believed that this disease outbreak resulted when the chronically immuno-suppressed fish population experienced an additional stressor(s) in 1994/95 that overcame the capabilities of the fish to manage bacterial pathogens shown to be present in both reference and exposed fish populations. Some combination of chemicals in the mill effluent, sewage effluent or natural environmental factors precipitated the problem. This conclusion led to the decision to divert

the sewage effluent (to the mill effluent treatment and tailings management circuit) in a further attempt to reduce fish stress (commencing January 1998).

Ongoing non-fatal visual fish health surveys and a tagging programme (199 fish) were implemented in June 1997 to track disease progression and a multi-year monitoring programme was implemented to gauge the impact of sewage discharge into the same drainage system.

In 2000, reports were filed on these two 1997 monitoring initiatives [6,7]. Results are briefly summarized below:

— The data from the three-year monitoring survey continues to support the conclusion that the fish populations in the near-field waterbodies of the David Creek drainage experienced a severe bacterial disease outbreak some time in 1995. Previous findings and the results of the three-year survey suggest that the population has been gradually recovering from this initial outbreak of ulcerative skin disease as both disease intensity and prevalence have markedly decreased;

— Comparisons restricted to the spring portion of the three-year monitoring period show that the percent disease incidence has declined from 25% in 1997 to a low of 3.3% in 1999. The severity of lesions has also markedly decreased as shown by comparisons of the intensity of the ulcerations in the early outbreak years relative to those observed in 1999. This decrease in disease incidence and intensity corresponds to the increase in overall fish health over the last two years. In the last two years, the overall physical condition of individuals in the population has improved;

— The low capture mortality (especially for white suckers) and high recapture success within and between sample years indicate that the programme was successful in minimizing study related mortality and that fish with ulcerations can recover from the disease. The number of fish with scars from previous disease expression has fluctuated, but provides clear evidence that fish with lesions can recover and survive through multiple years. Some of these healed lesions were quite large indicating that individuals recover from severe ulcerations;

— Despite this evidence of recovery this population is likely still at risk of another bacterial disease outbreak should they be exposed to any additional stressors, whether anthropogenic or natural; and

— Raven Lake, which received the treated sewage effluent prior to diversion in January 1998, is expected to recover very slowly, due to the low flushing rate for the lake.

Availability of nutrients from sediments results in substantially higher phytoplankton productivity relative to a nearby reference lake. The zooplankton community in Raven Lake has responded with a complimentary increase in its own productivity along with a corresponding decrease in community complexity. The benthic macroinvertebrate community also exhibited the increased productivity/decreased community complexity trend. The only biotic component that does not show increased productivity in Raven Lake is the aquatic macrophyte community, likely due to dense algae blooms which are limiting summer time light penetration. This current work was done primarily to provide baseline for future decommissioning phase recovery evaluation.

Studies of fish health in the near-field waters receiving mill effluent have not yet been brought to closure. In July and August 2000, monthly rainbow trout acute toxicity tests of the discharge from Wolf Lake (the small, 50 000 m³ lake which first receives treated mill effluent) failed, despite the fact that actual end-of-pipe effluent samples passed both trout and

Microtox tests. This apparent anomaly is pH-related. Wolf Lake shows pH depression from about six in the winter to four in the summer. These were the first toxicity failures in five years of data collection from this point in the near-field receiving environment. Ammonia oxidation (nitrification) from NH3 and NH4 to nitrate/nitrite is undoubtedly a major factor in pH depression in the summer months. However, pH starts to drop in the March-May period, when water temperature is too low for rapid ammonia oxidation to occur. Therefore it appears that there might be more than one factor involved. Acid-rain spring melt runoff, low pH releases from the vicinity of the above-ground tailings management facility, and sediment degradation have all been suggested as possible additional causative agents.

The fact that we are currently studying pH suppression was expected. In our 1998 progress report to this CRP, we predicted follow up on the following two issues in relation to fish health:

(i) Investigation of the cause of pH suppression in the receiving water. Natural pH levels in these waters are in the pH 6–7 range. Values as low as 3.65–3.75 have been recorded in effluent receiving water bodies. Acidic water conditions of less than five are known to be stressful to white suckers. A pH data logging system was proposed to monitor seasonal effects. The water appears to drop in pH downstream from the effluent discharge point. Effluent is typically discharged at pH 6-6.5; and

(ii) Sediment contaminant bioavailability studies. In light of as-yet unexplained pH effects, and likely impact on sediment contaminant behaviour, further study was felt warranted.

We were unable to carry out the pH work in 1999. The Key Lake mill was shut down (for retrofitting to receive McArthur River ore), thus it was an atypical year. It was however noted that pH data in the summer of 1999 was not as depressed as previous summer periods when full volume mill effluent was being discharged. Formal regulatory requests for additional sediment toxicity testing as follow up to the 1997 – 1999 work have recently been received.

The 1999 sediment toxicity work [5] looked at the small lake downstream of Wolf Lake, where the near-field benthic macroinvertebrate community has been impacted by treated mill effluent. The next step is likely a sediment TIE study.

6. Nickel loading reduction initiatives