Microbial assembly and co-occurrence network in an aquifer under press perturbation

Purpose

Thousands of aquifers worldwide have been polluted by leachate from landfills and many more remained threatened. Microbial communities from these environments play a crucial role in mediating biodegradation and maintaining the biogeochemical cycles, but their co-occurrence and assembly mechanism have not been investigated.

Method

Here, we coupled network analysis with multivariate statistics to assess the relative importance of deterministic versus stochastic microbial assembly in an aquifer undergoing intrinsic remediation, using 16S metabarcoding data generated through Illumina MiSeq sequencing of the archaeal/bacterial V3–V4 hypervariable region.

Results

Results show that both the aquifer-wide and localised community co-occurrences deviate from expectations under null models, indicating the predominance of deterministic processes in shaping the microbial communities. Further, the amount of variation in the microbial community explained by the measured environmental variables was 55.3%, which illustrates the importance of causal factors in forming the structure of microbial communities in the aquifer. Based on the network topology, several putative keystone taxa were identified which varied remarkably among the wells in terms of their number and composition. They included Nitrospira, Nitrosomonadaceae, Patulibacter, Legionella, uncharacterised Chloroflexi, Vicinamibacteriales, Neisseriaceae, Gemmatimonadaceae, and Steroidobacteraceae. The putative keystone taxa may be providing crucial functions in the aquifer ranging from nitrogen cycling by Nitrospira, Nitrosomonadaceae, and Steroidobacteraceae, to phosphorous bioaccumulation by Gemmatimonadaceae.

Conclusion

Collectively, the findings provide answers to fundamental ecological questions which improve our understanding of the microbial ecology of landfill leachate plumes, an ecosystem that remains understudied.

A nature-based solution to a landfill-leachate contamination of a confined aquifer

Remediation of groundwater from landfill contamination presents a serious challenge due to the complex mixture of contaminants discharged from landfills. Here, we show the significance of a nature-based solution to a landfill-contaminated aquifer in southeast Norway. Groundwater physicochemical parameters monitored for twenty-eight years were used as a proxy to infer natural remediation. Results show that concentrations of the major chemical variables decreased with time and distance until they tailed off. An exception to this was sulphate, which showed an increase, but apparently, exhibits a stationary phase. The water types were found to be most similar between samples from active landfill and post-closure stages, while samples from the stabilised stage showed a different water type. All the chemical parameters of samples from the stabilised stage were found to be within the Norwegian drinking water standards, except iron and manganese, which were only marginally above the limits, an indication of a possible recovery of this aquifer. The findings highlight the significance of natural attenuation processes in remediating contaminated aquifers and have significant consequences for future contamination management, where natural remediation can be viewed as an alternative worth exploring. This is promising in the wake of calls for sustainable remediation management strategies.

Spatiotemporal and seasonal dynamics in the microbial communities of a landfill-leachate contaminated aquifer

The microbiome of an aquifer contaminated by landfill leachate and undergoing intrinsic remediation was characterised using 16S rRNA metabarcoding. The archaeal/bacterial V3-V4 hypervariable region of the 16S rRNA gene was sequenced using Illumina MiSeq, and multivariate statistics were applied to make inferences. Results indicate that the aquifer recharge and aquifer sediment samples harbour different microbial communities compared to the groundwater samples. While Proteobacteria dominated both the recharge and groundwater samples, Acidobacteria dominated the aquifer sediment. The most abundant genera detected from the contaminated aquifer were Polynucleobacter, Rhodoferax, Pedobacter, Brevundimonas, Pseudomonas, Undibacterium, Sulfurifustis, Janthinobacterium, Rhodanobacter, Methylobacter and Aquabacterium. The result also shows that the microbial communities of the groundwater varied spatially, seasonally and interannually, although the interannual variation was significant for only one of the wells. Variation partitioning analysis indicates that water chemistry and well distance are intercorrelated and they jointly accounted for most of the variation in microbial composition. This implies that the species composition and water chemistry characteristics have a similar spatial structuring, presumably caused by the landfill leachate plume. The study improves our understanding of the dynamics in subsurface microbial communities in space and time.

Long-term redox conditions in a landfill-leachate-contaminated groundwater

Indicators of redox conditions; oxygen, sulphate, nitrate, ammonium, iron and manganese, and in addition, bicarbonate and total organic carbon were studied in groundwater samples contaminated by leachate emanating from Revdalen Landfill (Norway). Based on these variables, the study aimed to deduce the redox conditions in the aquifer. Literature on landfill leachate contamination of confined aquifers is scarce and to the best of our knowledge, this study, which describes long-term analysis of redox chemistry, is the first of its kind in such an environment. Groundwater samples were monitored for a period of 24 years, enabling us to describe redox conditions on both short-term and long-term bases. Levels of measured parameters in the contaminated aquifer varied spatially and with time, but were generally elevated except oxygen; pH (4.9-8.8), oxygen (0-11.3 mg/L), sulphate (0-28 mg/L), nitrate (0-16 mg N/L), ammonium (0.02-40 mg/L), iron (0-99 mg/L), manganese (0.06-16 mg/L), bicarbonate (22-616 mg/L) and total organic carbon (1.3-47 mg/L). From the result, levels of iron, manganese, nitrate and ammonium violated the Norwegian drinking water norms. However, iron, ammonium, total organic carbon and bicarbonate showed strong attenuation along the groundwater flow path. By contrast, oxygen, nitrate and sulphate increased farther out in the plume. The redox conditions that developed in the aquifer were similar to those previously reported for phreatic aquifers, structuring by proximity to the landfill as sulphate-reducing, iron-reducing, manganese-reducing, nitrate-reducing, and finally aerobic condition. Eventually, there was an apparent breakdown of this system due to ecosystem shift in the landfill when leachable reduced ions were depleted and the landfill became aerobic. Overall, the redox framework provided remarkable attenuation to contaminants, and thus prevented potential degradation of ecological health due to the landfill leachate.

Groundwater contamination from a municipal landfill: Effect of age, landfill closure, and season on groundwater chemistry

Groundwater reservoirs continue to be threatened globally, mainly from anthropogenic activities. There is need to understand how remediation of groundwater can be influenced by site-specific factors. There are few studies, if any, that incorporate at least three site-specific factors in a single investigation of groundwater contamination from landfills. We report a study where waste age, landfill closure, and season were compared with changes in water quality, using a twenty-four-year groundwater chemistry dataset. Groundwater samples were extracted from monitoring wells and analysed for twenty-eight physicochemical parameters. Results showed discharge of both legacy pollutants and elevated inorganic pollutants into the groundwater. Among the site-specific factors, waste age was the most influential. At the landfill age of 21 years, concentrations of pollutants became close to the reference value. The result also indicated that closing the landfill caused significant decrease in concentrations of contaminants in the groundwater (P < 0.05). Season was the least influential, registering significant results only for dissolved oxygen, sulphate and chloride (P < 0.05). Lastly, the result showed strong attenuation of pollutants with distance, thereby demonstrating the feasibility of the aquifer acting as a natural treatment plant to the pollutants. This eliminates any serious environmental risk associated with the emanating leachate, but at a cost of prohibiting abstraction of the groundwater for human use, due to potential health risks.