Article by Sanda Iepure, Groundwater Ecology Group

Sanda Iepure, Groundwater Ecology Group, IMDEA Agua

Bearing in mind a holistic conceptual perspective of groundwater protection as a natural resource and as an living system, and in response to the increasing concern regarding the occurrence, fate and effect of man-made products, the current groundwater environmental risk assessments (ERA) require new approaches. Therefore, the ecotoxicity tests on target species is a groundbreaking solution that generate substantial outcomes linked to the effects of specific compounds on groundwater aquatic life forms, besides the through identifications and characterisation of the potential risks that pose the human activities on aquifers. However, the target organisms for ecotoxicity tests has to be selected with caution as to be relevant for groundwater ecosystems.

Groundwater is an important resource on earth which provide vital water supplies for human use. Groundwater also host a significant component ot total worldwide aquatic ecosystems biodiversity (Gibert et al., 1994; Deharveng et al., 2011) that plays a crucial role in maintaining water quality (Danielopol et al., 2003; Griebler et al., 2013). Continuing water scarcity, climate change pooled to human contamination (uncontrolled urban, rural and industrial discharges, cross-contamination of aquifers, etc.) place these ´hidden´ reserves at high risk in terms of sustainability of their functions.

The groundwater are protected under the general umbrella of Water Framework Directive (WFD, 2000/60/EC) and specifically by the Groundwater Framework Directives (GWD, 2006/118/EC), however the aquifers as ecosystems are still hardly considered. The WFD claims to the member states to achieve the good quality and quantity status and the GWD complement it by establishing a set of quality standards and measures to prevent and limit the input of contaminants into aquifers. The GWD comprise a list of pollutants and environmental standards (in the Annex II) for which the member states should establish threshold values. But the list is so far extremely short and currently includes only 10 substances/ions or indicators (As, Cd, Pb, Hg, ammonium, chloride, sulphate, Trichloroethylene, Tetrachloroethylene, Conductivity). Several man-made products detected in the last decade in aquifers (especially in the shallow ones) are completely missing i.e. pesticides, ammonia, volatile organic compounds (VOC) or emerging contaminants like pharmaceutical and personal care products (EEA, European Environmental Agency, 2007). The presence of these compounds in high concentrations in aquifers would lead not only to a decline of water quality and biodiversity loss, but would increase the costs of groundwater rehabilitation, which is in any case lenghtly and sometime impossible to be achieved.

Currently, it is recognized that the water quality guidelines applied to surface waters ecosystems is not completely reliable when dealing with groundwater, as they are completely distinct ecosystems although interconnected. Moreover, some contaminants wich are of relevance for surface waters are not always the most important in groundwater. The pollution risk imply a real or a potential consequence on the health of the ´ecological´ receptor arising from a source point via subsurface flowpaths (Fig. 1). When dealing with groundwater a receptor is associated with a drinking resource, an aquifer or a groundwater dependent ecosystem i.e. wetlands, hyporheic zone of rivers. So far the groundwater risk assessment use qualitative descriptors to primarily evaluate the water storage, use and impacts of hazards on water availablity and quality. It comprise the evaluation of source contamination on a site and the potential risk to a water course or an aquifer. The assessment of groundwater pollution threats is currently approached by direct methods via monitoring and the traditional analytical detection of chemical elements indicative for water quality, presence of contaminants and incipient degradation of the groundwater bodies; and by indirect methods comprising estimation of groundwater contaminants loads and vulnerability to pollution of the groundwater body in question. But in the last decade, the conceptual frameworks of groundwater (or groundwater dependent ecosystems) risk assessments slowly starts to provide methods to appraise the ecological value of subterranean aquatic ecosystems (i.e. Malard et al., 1996; Steube et al., 2008; Swiss Water Protection Ordinance, 1998; Western Australian Guidance, 2003, EU-GWD 2006). Nevertheless, beside the ecological assessment scheme that include the description of aquifers typologies (groundwater ecosystems), definition of reference status (natural background values), identification of bioindicators and the evaluation model (Griebler et al., 2010, Steube et al, 2014), a finest practice to obtain a reliable ecological status of groundwater ecosystems are the ecotoxicological assessments by using typical subterranean-dweller species.  

WHAT GROUNDWATER ECOTOXICOLOGY TESTS INFORM US?

The ecotoxicity bioassays imply the detection of a response of a species to the exposure of a substance or to a concentration of a particular constitution of a mixture on short (acute) or long-term (chronic). This characterisation also imply the evaluation of organisms resilience – how fast an organism/population recover in its enviroment, which are the key factors to mantain the populations sustainability to warrant the ecosystem functioning. The impacts measured in ecotoxicity tests can be tested at distinct level of organisation (i.e. individual species, populations, communities, ecosystem) and includes: mortality, sublethal endpoints processes (feeding behaviour, osmoregulatory and respiration rate, mobility), growth reduction, reproductive impairement, species turnover and changes in species number in a community, bioaccumulation of compounds in target species, disrumption of community and ecosystem-level functions. The ecotoxicity tests also imply the characterisation of an ecological effect that describe how toxic is a contaminant for distinct species and or other ecological entities (communities), what kind of effects produces, how to relate the effects to endpoints and ultimately, how the effects changes with distinct levels of contaminant exposure.

The contaminants are continuously released to the groundwater and the fauna may therefore be systematically exposed to high concentrations of chemicals and experience serious toxic stress. The contribution of ecotoxicity tests in groundwater ERA have specific outcomes refering to distinct stages of the assessment to primarily detect if the concentration of a contaminant is high enough to cause an adverse effect for species. Furthermore, the tests offer informations related to the bioavailability of stressors, aggregate toxic effects of several stressors, development of new toxicity informations, defining the nature of toxic effects, distribution of toxicity and biomonitoring.

i)        Bioavailability of the contaminant. The simple presence of a contaminant in groundwater is not necessarily to have an adverse effect for an organism, only if occur in a bioavailable form. For example As is one of the most frequent natural element in several aquifers crustal rocks (Gómez et al., 2006). In general, reduced inorganic As found in sulfide minerals is relatively low in toxicity, but oxidized inorganic As(III) and As(V) compounds are two or three times more toxic than many organoarsenicals.   

ii)      Aggregate toxic effects of several contaminants. Several aquifers are exposed to a complex of contaminants and than just chemical alone could not predict the potential toxicity upon organisms. Hence, toxicity tests are of huge wealth to measure the individual effect of a compound or a mixture of them first in controled laboratory conditions and thereafter in situ.    

iii)     Evaluate the toxicity of substances of which biological effects is less known. Several substances present in aquifers has been just recently detected in aquifers and their toxic effects on aquatic life has been only recently investigated (Avramov et al., 2013; Di Marzio et al., 2013; Di Lorenzo et al., 2014, 2015). The toxicity tests in this case are informative to distinguish the specific problems of an aquifer. For example the ecotoxicity protocol for testing the effects of VOCc compounds from mineral oil (i.e. benzene, toluene, ethylben-zene, o-, p- and m-xylene) and chlorinated hydrocarbons on groundwater species has been just recently developed (Avramov et al., 2013).

iv)     Detect the toxic effect. Toxicity tests are informative to detect if a contaminant has an lethal or sublethal effects on selected species. The toxicity tests can measure lethal or sublethal effects on organisms which are called endpoints. In acute toxicity tests the endpoints can be the mortality which is calculated by comparing a % of exposed organisms to a media to the % of organisms exposed to uncontaminated media (usually performed in an interval of 24 to 96 h) or the estimation of the dilution of the medium at which 50% of oganisms died (LC₅₀). Alternatively it might be also calculated the half maximal effective concentration (EC₅₀) that refer to the concentration of a toxic compound inducing response to 50% of the maximal possible effect from a an organism. In chronic tests (on long-term) are measured lethal and sublethal effects. The later includes evaluations of growth reduction, reproductive impairement, mobility lack, or inhibition of some regulatory functions such are development, fertility, changes in the behaviour and mantainance of homeostazis (Crisp et al., 1998).

v)      Characterize the distribution of toxicity at a site. For a selected aquifer the toxicity tests might be performed at different site-location and in this case it might be determined the spatial extent of toxicity within an aquifer and the specific areas with high toxicity levels.

vi)     Remediation actions and monitoring. Although the aquifers remediation (or restoration) is a difficult process (involving physical or chemical treatment, biological treatment, or electrokinetics) toxicity tests might be informative for remedial goals. Depending by the level of contaminant toxicity, a goal might be to reduce the toxicity for a period of time. Further, the remediation process might be monitored which is indicative to detect if the sources of contamination are still present and if remediation measures to reduce toxicity are efficient for supporting a viable ecological community.

vii)   Bioindicators for monitoring groundwater ecosystem health. Groundwater species once selected to be used in toxicity tests are valuable bioindicators for groundwater health monitoring and in identifying and adjusting the water quality standards (threshold values) for specific compounds.  

WHY AND HOW TO USE GROUNDWATER SPECIES IN ECOTOXICITY TESTS?

The ecotoxicity tests in ecological risk assessment involve the identification of chemical threats to environment by studing their toxic effects on target organisms. For surface waters the standard organisms tests are algae, fishes and aquatic invertebrates. For groundwater, in the absence of the first two groups, the target species are invertebrates and among them the crustaceans. Subterranean crustaceans are ecologically of three types, stygoxenes (species transported accidentaly in groundwater by surface water infiltration), stygophyles (surface species but with abilities to leave in groundwater part of their life-cycle) and stygobionts (obligate dwellers which lives exclusively in groundwater) (Gibert et al., 1994). The stygobites as true groundwater dwellers have significant atributes making them suitable for toxicity bioassays (i.e. small size, short life-cycle, relatively easely rearing in case of some species) (Di Marzio et al., 2009, 2013). When compared to surface species the stygobites shows specific adaptations to fit in the specific habitat conditions and harsh life in the subterranean waters: morphologic characters (i.e. blind, non-pigmented, small and elongated body shape), metabolic (i.e. low metabolic rate, long life, low reproduction rate, resistant to hypoxia and to limited food supply etc.) and ecological (i.e. narrower tolerance range being stenotopic with the necessity of strict environmental conditions, low dispersion capacity). Because of these strict specializations they are highly sensitive to any disturbance in their environment (both quality and quantity).

One of the critical issue in groundwater ecology tests is that standard ecotoxicological bioassays applied to surface water organisms do not seem apprropiate for groundwater species. Hence the primary steps is the development of the protocol and selecting the most appropiate endpoints. Previous ecotoxicity tests using stygobites as target species suggest that are significant differences in sensitivity among surface and groundwater species even if they are taxonomically relatives (i.e. species from the same genera). The laboratory tests indicated for example that stygobites isopod Proasellus cavaticus are more tolerant to Cr and Cu than the surface species Proasellus coxalis, whereas for KCl and KNO₃ the the former are more sensitive. The harpacticoids Parastenocaris germanica appear highly sensitive to fungicide the Thiram, ammonia and aldicarb that alter post-naupliar development in acute tests. It is suggested that a longer exposure of the stygobites harpaticoids to high concentrations on ammonia and aldicarb (> than used in the tests, 19 mg/L of ammonia and > 3 mg/L for aldicarb) would probably cause a detrimental effect on populations attributes, such are age structures and abudance. Toluen was harmful for the surface cladocera Ceriodaphnia dubia and the amfipod Gammarus minus at concentration of 7 mg/L^-1 and 58 mg/L^-1 and it was toxic for the stygobite Niphargus inopinatus at much higher values for example 100% mortality was obtained at values of 118.7 mg/L^-1) (Avramov et al., 2013; Di Marzio et al., 2013; Di Lorenzo et al., 2015).

Within REMTAVARES project (Programa de I+D Comunidad de Madrid, S2013/MAE-2716) the IMDEA Agua group aims to design a protocol test for evaluating and determine the sensitivity of surface and typical groundwater invertebrate species to emerging compounds specifically pharmaceuticals, personal care products and trace metals. We specifically aims to tackle the lethal and sublethal concentrations of selecte contaminants on both surface and groundwater crustacean invertebrates and assess the toxic stress effects on their physiologic (i.e. oxygen consumption) and metabolic rates. Our study targets to contribute to current attempts in establishing threshold values for emerging compounds in surface waters (rivers) and groundwaters and to advance the assessment methods to asess the ecological risk in aquatic ecosystems. 

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