Groundwater-fed Streams Research Articles

Drawdown "Triggers": A Misguided Strategy for Protecting Groundwater-Fed Streams and Springs.

Article impact statement: Protecting groundwater‐dependent ecosystems near mining developments requires more than setting drawdown “trigger levels.”

Analysis of metolachlor ethane sulfonic acid (MESA) chirality in groundwater: A tool for dating groundwater movement in agricultural settings

To better address how much groundwater contributes to the loadings of pollutants from agriculture we developed a specific dating tool for groundwater residence times. This tool is based on metolachlor ethane sulfonic acid, which is a major soil metabolite of metolachlor. The chiral forms of metolachlor ethane sulfonic acid (MESA) and the chiral forms of metolachlor were examined over a 6-year period in samples of groundwater and water from a groundwater-fed stream in a riparian buffer zone. This buffer zone bordered cropland receiving annual treatments with metolachlor. Racemic (rac) metolachlor was applied for two years in the neighboring field, and subsequently S-metolachlor was used which is enriched by 88% with the S-enantiomer. Chiral analyses of the samples showed an exponential increase in abundance of the S-enantiomeric forms for MESA as a function of time for both the first order riparian buffer stream (R2=0.80) and for groundwater within the riparian buffer (R2=0.96). However, the S-enrichment values for metolachlor were consistently high indicating different delivery mechanisms for MESA and metolachlor. A mean residence time of 3.8years was determined for depletion of the initially-applied rac-metolachlor. This approach could be useful in dating groundwater and determining the effectiveness of conservation measures. One sentence summaryA mean residence time of 3.8years was calculated for groundwater feeding a first-order stream by plotting the timed-decay for the R-enantiomer of metolachlor ethane sulfonic acid.

Flow intermittency, physico-chemistry and function of headwater streams in an Alpine glacial catchment

Relatively little is known regarding the extent of intermittent streams or the general ecology of headwaters in alpine catchments with glacial influence. This study quantified the contribution of intermittent streams to the total length of the stream network along with an ecological assessment during spring-summer of headwater streams (higher than 1,900 m above sea level) in the Val Roseg, a high Alpine glacial catchment. Stream network mapping revealed that ca. 90 % (76.8 km) of the drainage network consisted of intermittent streams. Glacier-fed headwaters experienced diel surface flows in late spring and summer, most going dry during the night due to reduced glacial inputs. In contrast, groundwater-fed streams often went dry in summer with the contraction of groundwater and other subsurface inputs. A principal components analysis of physico-chemical characteristics revealed headwaters to be primarily glacial-fed (kryal), groundwater-fed (krenal), or having a mixed water source. Although quite variable, periphyton biomass reached high levels (ca. 40 mg m−2 chl-a, 10 g m−2 AFDM) by late spring in most headwaters. Organic matter in transport (seston) ranged from 0.03 to 0.09 mg L−1 mostly consisting of fine particulate organic matter (FPOM: 33–76 %). Hyporheic sediment respiration rates varied considerably, ranging from 0.005 to 0.126 mg O2 h−1 kg−1 sediment and primarily related to the amount of loosely attached organic matter. These results indicate that intermittent streams are predominant in alpine landscapes, comprising mostly 1st to 2nd order systems, and that ecosystem properties vary substantially among headwater streams likely in relation to annual/daily changes in flow and water source. Such headwaters may contribute strongly to the production, processing and transport of organic matter to downstream waters, especially in light of the expected increase in intermittent streams in alpine catchments experiencing rapid glacial recession.

Comparing the groundwater contribution in two groundwater-fed streams using a combination of methods

Groundwater contributions to streams often vary both spatially and temporally. In this study, the relative contributions of groundwater to Fishtrap and Bertrand Creeks in the Lower Fraser Valley are compared during the summer and annually using a combination of methods. The streams drain watersheds that have a similar climate, area and topographic relief. However, the geological substrate differs, with finer grained material underlying Bertrand Creek. A simplified heat budget model is first used to estimate temperature at the air–water interface. Then, the correlation between the air–water and bed–water (interface) temperatures is compared for both annual and summer periods in each stream. The air temperature explains ~30% of the variance in the summer interface temperature in Fishtrap Creek, but more than 75% in Bertrand Creek. This difference suggests that factors at the bed–water interface influence the heat budgets, and that these factors differ between the two sites. The measured groundwater flux during the summer periods was higher in Fishtrap Creek than in Bertrand Creek, due to a higher vertical hydraulic gradient into the stream and higher aquifer sediment permeability. Thus, the larger groundwater contribution to Fishtrap Creek moderates stream temperature and stream discharge during the summer periods, whereas Bertrand Creek has greater potential for elevated stream temperatures and lower flows during the critical summer period The combination of methods used in this study strengthens the interpretations given the wide range of uncertainty in measurements.

Changes in invertebrate assemblage composition in benthic and hyporheic zones during a severe supraseasonal drought

AbstractDroughts are unpredictable disturbances characterized in streams by declining flow, reduced habitat availability, and deteriorating abiotic conditions. Such events typically reduce benthic invertebrate taxon richness and modify assemblage composition, but little is known about how hyporheic invertebrate assemblages respond to drought or how these responses relate to changes in benthic assemblages. We hypothesized that taxon richness (α diversity) and variability (as within-site β diversity) in benthic assemblage composition would decline as drought proceeded, whereas concurrent changes in hyporheic assemblages would be lower in this more stable environment. We predicted that benthic assemblage composition between sites would converge as epigean taxa were selectively eliminated, whereas between-site hyporheic β diversity would change little. We sampled benthic and hyporheic invertebrates concurrently from 4 sites along a groundwater-fed stream during the final stages of a severe supraseasonal droug...

Spatio-temporal patterns of major bacterial groups in alpine waters.

Glacial alpine landscapes are undergoing rapid transformation due to changes in climate. The loss of glacial ice mass has directly influenced hydrologic characteristics of alpine floodplains. Consequently, hyporheic sediment conditions are likely to change in the future as surface waters fed by glacial water (kryal) become groundwater dominated (krenal). Such environmental shifts may subsequently change bacterial community structure and thus potential ecosystem functioning. We quantitatively investigated the structure of major bacterial groups in glacial and groundwater-fed streams in three alpine floodplains during different hydrologic periods. Our results show the importance of several physico-chemical variables that reflect local geological characteristics as well as water source in structuring bacterial groups. For instance, Alpha-, Betaproteobacteria and Cytophaga-Flavobacteria were influenced by pH, conductivity and temperature as well as by inorganic and organic carbon compounds, whereas phosphorous compounds and nitrate showed specific influence on single bacterial groups. These results can be used to predict future bacterial group shifts, and potential ecosystem functioning, in alpine landscapes under environmental transformation.

Bacterial structures and ecosystem functions in glaciated floodplains: contemporary states and potential future shifts

Glaciated alpine floodplains are responding quickly to climate change through shrinking ice masses. Given the expected future changes in their physicochemical environment, we anticipated variable shifts in structure and ecosystem functioning of hyporheic microbial communities in proglacial alpine streams, depending on present community characteristics and landscape structures. We examined microbial structure and functioning during different hydrologic periods in glacial (kryal) streams and, as contrasting systems, groundwater-fed (krenal) streams. Three catchments were chosen to cover an array of landscape features, including interconnected lakes, differences in local geology and degree of deglaciation. Community structure was assessed by automated ribosomal intergenic spacer analysis and microbial function by potential enzyme activities. We found each catchment to contain a distinct bacterial community structure and different degrees of separation in structure and functioning that were linked to the physicochemical properties of the waters within each catchment. Bacterial communities showed high functional plasticity, although achieved by different strategies in each system. Typical kryal communities showed a strong linkage of structure and function that indicated a major prevalence of specialists, whereas krenal sediments were dominated by generalists. With the rapid retreat of glaciers and therefore altered ecohydrological characteristics, lotic microbial structure and functioning are likely to change substantially in proglacial floodplains in the future. The trajectory of these changes will vary depending on contemporary bacterial community characteristics and landscape structures that ultimately determine the sustainability of ecosystem functioning.

Air‐Water Temperature Relationships in the Trout Streams of Southeastern Minnesota's Carbonate‐Sandstone Landscape

AbstractCarbonate‐sandstone geology in southeastern Minnesota creates a heterogeneous landscape of springs, seeps, and sinkholes that supply groundwater into streams. Air temperatures are effective predictors of water temperature in surface‐water dominated streams. However, no published work investigates the relationship between air and water temperatures in groundwater‐fed streams (GWFS) across watersheds. We used simple linear regressions to examine weekly air‐water temperature relationships for 40 GWFS in southeastern Minnesota. A 40‐stream, composite linear regression model has a slope of 0.38, an intercept of 6.63, and R2 of 0.83. The regression models for GWFS have lower slopes and higher intercepts in comparison to surface‐water dominated streams. Regression models for streams with high R2 values offer promise for use as predictive tools for future climate conditions. Climate change is expected to alter the thermal regime of groundwater‐fed systems, but will do so at a slower rate than surface‐water dominated systems. A regression model of intercept vs. slope can be used to identify streams for which water temperatures are more meteorologically than groundwater controlled, and thus more vulnerable to climate change. Such relationships can be used to guide restoration vs. management strategies to protect trout streams.

Response of Terrestrial-Aquatic Palm Ecotone (Morichal) to Anthropogenic Disturbance in the Orinoco Lowlands

The study tested the assumption that the effect of land-use changes on hydrological dynamics and edaphic features of an aquatic-terrestrial ecotone have led to vegetational patchiness and decrease primary productivity (NPP). On the basis of the depletion of a groundwater-fed stream, three study sites corresponding to interrupted, intermittent and permanent streams were selected throughout the ecotone in the Sunsunes catchment (Orinoco basin, Venezuela). To describe the human impact on land cover, patchiness, biodiversity, hydrological and edaphic features, NPP and nutrient availability, we use structural and functional approaches. Hydrological (i.e., duration of inundation and maximum inundation height), soil chemical (e.g., Ca concentration, available phosphorous, soil organic matter) and physical (i.e., water-filled pore spaces) features were the best predictors of anthropogenic disturbance. In the ecotone, the tree species invasion from well-drained savannas increased woody cover as described by a stretched exponential model. Groundwater drawdown in the interrupted and intermittent streams increased with 74 and 34 colonizer species from well-drained savannas. The NPP of the ecotonal vegetation along the interrupted stream (909 g C/m2/yr) was a higher sink as compared to the intermittent and permanent streams (580 g C/m2/yr). Anthropogenic stress along with natural disturbance resulted in a decline in the system´s functioning. In contrast to hydrology, the effect of the nutrient addition (i.e., liming and phosphorous) on the carbon accumulation by species was not significant. Therefore, the functional response of the system was more sensitive to hydrology regime. The results indicate impact on the ecotones occurred in short term, and that vulnerability to climate is crucial of groundwater-dependent vegetation.

Water Flow Dynamics of Groundwater-Fed Streams and Their Ecological Significance in a Glacierized Catchment

Subsurface flow pathways of groundwater-fed streams were characterized on a floodplain terrace of the Toklat River, Alaska, in summer 2008, to establish the influence of local physicochemical variability upon macroinvertebrate communities. Streams proximal to the valley side (A sites) and to the main meltwater channel (B sites) were studied. Chloride and natural isotopic tracers (δ18O and δ2H) were used to identify water sources and flow pathways. Results indicated that flow in B sites was dominated by seepage of glacial meltwater through the alluvial aquifer. Streamflow at sites situated at higher elevations was ephemeral, and commenced with the seasonal rise in the groundwater table. In contrast, the physicochemistry of A sites was characteristic of seepage from valley-side debris fans, which maintained perennial flow to streams at lower elevations. Macroinvertebrate diversity was lower in ephemeral streams, likely due to colonization constraints. In June macroinvertebrate abundance was significantly positively correlated with the percentage contribution to streamflow from debris-fan seepage (p < 0.05) and with fine particulate organic matter concentration (FPOM) (p < 0.05); FPOM was correlated with debris fan seepage (p < 0.05). These relationships were not evident in July and August, when organic matter availability increased. Our study demonstrates that flow pathways and organic matter availability significantly influence macroinvertebrate communities in these groundwater-fed streams.

Coevolution of hydrology and topography on a basalt landscape in the Oregon Cascade Range, USA

AbstractYoung basalt terrains offer an exceptional opportunity to study landscape and hydrologic evolution through time, since the age of the landscape itself can be determined by dating lava flows. These constructional terrains are also highly permeable, allowing one to examine timescales and process of geomorphic evolution as they relate to the partitioning of hydrologic flowpaths between surface and sub‐surface flow. The western slopes of the Cascade Range in Oregon, USA are composed of a thick sequence of lava flows ranging from Holocene to Oligocene in age, and the landscape receives abundant precipitation of between 2000 and 3500 mm per year. On Holocene and late Pleistocene lava landscapes, groundwater systems transmit most of the recharge to large springs (≥0·85 m3 s−1) with very steady hydrographs. In watersheds &gt;1 million years old, springs are absent, and well‐developed drainage networks fed by shallow subsurface stormflow produce flashy hydrographs. Drainage density slowly increases with time in this basalt landscape, requiring a million years to double in density. Progressive hillslope steepening and fluvial incision also occur on this timescale. Springs and groundwater‐fed streams transport little sediment and hence are largely ineffective in incising river valleys, so fluvial landscape dissection appears to occur only after springs are replaced by shallow subsurface stormflow as the dominant streamflow generation mechanism. It is proposed that landscape evolution in basalt terrains is constrained by the time required for permeability to be reduced sufficiently for surface flow to replace groundwater flow. Copyright © 2010 John Wiley &amp; Sons, Ltd.

Macrophyte-based bioindication in rivers – A comparative evaluation of the reference index (RI) and the trophic index of macrophytes (TIM)

Submerged macrophyte vegetation has been mapped in four calcareous groundwater-fed streams in Bavaria (southern Germany) in order to compare and assess two different methods of river bioindication. The first one, the trophic index of macrophytes (TIM), is a tool to assess the trophic status of running waters. In contrast, the reference index (RI) is an ecological index which evaluates the difference between a reference community and the actual submerged vegetation, depending on the river type, as required by the Water Framework Directive. Water nutrient concentrations were measured once at selected sites in all water courses. The TIM reflects water phosphorus concentrations, accounting also for nutrients enrichment in the sediment, and is not influenced by shading, depth, substrate and flow velocity of the water course. The TIM is very sensitive to small variations in P concentration when the P level is low, while the index tends to a maximum as soluble reactive phosphorus (SRP) and total phosphorus (Ptot) exceed a certain value. The RI indicates river ecological status which is not only influenced by trophic status but by every factor leading to a deviation of the actual macrophyte community from the reference community. In the investigated rivers the RI indicated reduced flow velocity caused by milldams and shading by riparian vegetation, in addition to trophic status. In rivers that are at the boundary between two different river types, classification of river type can play a crucial role for river status assessment. Incorrect classification of river type can lead to both, a “too good” and “too bad” assessment.

Aquatic Invertebrates Of the Grand Staircase–Escalante National Monument, Utah

We use multiple years of collections in rivers, perennial wetlands, and ephemeral tinajas to report on overall biodiversity of aquatic invertebrates in the Grand Staircase–Escalante National Monument, Utah. A total of 570 samples of aquatic invertebrates was collected at 166 locations. Over the study period, invertebrates were identified from 31 orders, 104 families, and 192 genera. Major habitat types (rivers, perennial wetlands, and ephemeral tinajas) supported unique and taxonomically rich assemblages of invertebrates; taxonomic richness was greatest in rivers. Among rivers, richness of genera of aquatic invertebrates was greatest in groundwater-fed streams and perennial, snowmelt-runoff, rivers and least in flood-prone rivers. Future studies should focus on identifying and collecting invertebrates from unique habitats, especially the numerous wetland-like habitats that occur across the Grand Staircase–Escalante National Monument, such as hanging gardens and alcove pools, as well as ephemeral streams.

Low Flow Variability in Groundwater-Fed Streams

Accurate low flow measurements are important in groundwater-fed streams in order to assess in-stream flow requirements and the nature of stream interactions with groundwater, as well as the potential impacts of climate variability and climate change. Variability in stream discharge was measured over the low flow season in Bertrand Creek, Pepin Brook and Fishtrap Creek, which drain the Abbotsford-Sumas aquifer in southwest British Columbia and northwest Washington State. Discharge was measured to quantify variability: (1) along the length of the stream and relate those variations to differing groundwater contributions over the low flow season, and (2) across a series of cross-sections, both repeating across a middle cross-section (at-a-station measurements) and along consecutive sections from upstream to downstream (downstream measurements), to assess the repeatability of the low flow measurements. Downstream increases in discharge in Fishtrap Creek and Pepin Brook are attributed to groundwater discharging into the channels along these sections, as supported by lower dissolved oxygen concentrations and stable water temperatures. Along Bertrand Creek, stream water interaction with groundwater appears to be variable, with the stream recharging the aquifer along some reaches, and discharging in others. Over the summer season, discharge decreased in all streams; on average the decrease was in the range 0.01 to 0.05 m3/s. As the flow decreased, the coefficient of variation of the mean discharge tended to increase when the flow was very low (i.e., <0.1 m3/s), although the trends for Fishtrap were weak. Overall, the variations in measured discharge are of sufficient magnitude that subtle changes in streamflow caused by some real effect may be difficult to detect.

Stream–air temperature relations to classify stream–ground water interactions in a karst setting, central Pennsylvania, USA

Stream–ground water interactions in karst vary from complete losses through swallow holes, to reemergences from springs. Our study objective was to compare stream–air temperature and energy exchange relationships across various stream–ground water relationships in a carbonate watershed. It was hypothesized that ground water-fed stream segments could be distinguished from perched/losing segments using stream–air temperature relationships. Two types of computations were conducted: (1) comparisons of stream–air temperature relationships for the period of October 1999–September 2002 at 12 sites in the Spring Creek drainage and (2) detailed energy budget computations for the same period for ground water-dominated Thompson Run and Lower Buffalo Run, a stream with negligible ground water inputs. Weekly average air temperatures and stream temperatures were highly correlated, but slopes and intercepts of the relationship varied for the 12 sites. Slopes ranged from 0.19 to 0.67 and intercepts ranged from 3.23 to 9.07 °C. A two-component mixing model with end members of ground water and actual stream temperatures indicated that the slope and intercept of the stream–air temperature relationship was controlled by ground water inputs. Streams with large ground water inputs had greater intercepts and lesser slopes than streams that were seasonally losing, perched, and/or distant from ground water inputs. Energy fluxes across the air–water interface were greatest for the ground water-fed stream due to increased longwave, latent, and sensible heat losses from the stream in winter when large temperature and vapor pressure differences existed between the stream and air. Advection of ground water was an important source and sink for heat in the ground water-fed stream, depending on season. In contrast, along the seasonally losing stream reach, advection was of minimal importance and stream temperatures were dominated by energy exchange across the air- water interface. Overall, results showed that stream temperatures and energy exchange in this carbonate watershed were strongly related to stream–ground water interactions.

Prediction of macrophyte communities in drought-affected groundwater-fed headwater streams

The results of a 10 year study of groundwater-dominated streams in the chalklands of southern England show that macrophyte communities occupying the headwaters of such streams have a measurable variability in terms of species composition and spatial cover, especially in relation to river flow. This occurs as a result of species adaptation to typically ephemeral flow regimes. Associations have also been established between variations in the vegetation and hydroclimatic parameters at the catchment scale, such as rainfall, percolation and soil moisture conditions. This has led to the derivation of a system whereby the nature of headwater macrophyte communities can be forecast/hindcast, on the basis of parameterized, antecedent flow records, to a current accuracy level of 72% for any of 13 community types and 90% for any of four community groups. It is anticipated that this accuracy rate will improve with (i) an updated community classification, (ii) refinement of the parameterization procedures for flow records, and (iii) the extension of the method from the six study streams currently under scrutiny to a further 18 streams. Greater accuracy of prediction will be constrained by natural variation within the vegetation and the interaction of an array of geomorphological and land-use variables operating at diverse spatial scales. The process used here to establish vegetation–flow relationships could be used in other locations with groundwater-fed streams, providing a useful tool for assessing some of the impacts of bioclimatic changes driven by global warming. Copyright © 2005 John Wiley & Sons, Ltd.

Spatial and temporal water column and streambed temperature dynamics within an alpine catchment: implications for benthic communities

Despite the significant influence of temperature upon alpine stream benthic communities, thermal regimes of the water column and hyporheic zone of these mountain streams have received limited attention. This paper reports upon a detailed spatio-temporal study of water column and streambed temperatures undertaken within the Taillon–Gabiétous catchment, French Pyrénées, that aims: (1) to characterize the nature and dynamics of alpine stream water column and streambed thermal patterns; (2) to investigate stream thermal variability under a range of hydroclimatological conditions; and (3) to consider the implications of (1) and (2) for alpine stream benthic communities. The catchment contains four highly dynamic hydrological sources and pathways: (1) two cirque glaciers (Taillon and Gabiétous); (2) seasonal snowpacks; (3) a karst groundwater system; and (4) hillslope aquifers. Water column temperatures were monitored continuously at four sites located along the Taillon glacial stream and at three groundwater springs (two karstic and one hillslope) over the 2002 summer melt season. An eighth site (Tourettes) was established on a predominantly groundwater-fed stream with limited meltwater input. Bed temperatures (0·05, 0·20 and 0·40 m depth) and river discharge were measured at three sites: (1) the Taillon stream; (2) the Tourettes stream; and (3) below the confluence of (1) and (2). Air temperatures, incoming short-wave radiation and precipitation were recorded to characterize atmospheric conditions. Glacial stream water column temperatures increased downstream, although groundwater tributaries punctuated longitudinal patterns. Karstic groundwater streams were cooler and more thermally stable than the glacial stream (except at the glacier snout). Hillslope groundwater stream temperatures were most variable and, on average, the warmest of all sites. Streambed temperatures in the glacial stream were coldest and most variable whilst the warmest and least variable streambed temperatures were recorded in an adjacent groundwater tributary. Temperature variability was strongly related to: (1) dynamic water source and pathway contributions; (2) proximity to source; and (3) prevailing hydroclimatological conditions. The high thermal heterogeneity within this catchment may sustain relatively diverse benthic communities, including some endemic Pyrénéan macroinvertebrate taxa. Copyright © 2004 John Wiley & Sons, Ltd.

Relationship between sediment organic matter, bacteria composition, and the ecosystem metabolism of alpine streams

We tested whether sediment bacteria abundance (4’,6‐diamidino‐2‐phenylindole‐stained cell counts) were related to sediment organic content (ash‐free dry mass [AFDM]) in 11 nonforested streams of three different Alpine catchments during summer 2003. We used terminal restriction fragment‐length polymorphism (T‐RLFP, a molecular genetic technique) to test for seasonal and spatial differences in bacterial composition in these same streams. We then related the above parameters, in conjunction with periphyton biomass and hyporheic respiration, to whole stream estimates of gross primary production (GPP) and ecosystem respiration (ER) in a glacial and nonglacial stream, representing environmental extremes, in one of the catchments. The percentage of organic matter of sediments was 4–14% (0.01–0.04 g AFDM ml sediment−1), and counts of bacteria cells per millimeter of sediment averaged 2 × 106−4 × 106. Bacteria counts correlated with sediment AFDM only for streams in the catchment with highest sediment AFDM levels. Bacteria composition (based on the presence and absence of terminal restriction fragments from T‐RFLP analysis) changed seasonally in the different streams and differed between glacial—and groundwater‐fed streams. In the one catchment, hyporheic respiration averaged 0.0004 and 0.0003 g O2 h−1 kg sediment−1 and was positively correlated with AFDM (r2 = 0.23). Ecosystem metabolism displayed a strong seasonality, with GPP averaging 4.5 and 8.4 and ER averaging 5.4 and 9.9 g O2 m−2 d−1 for the two sites, respectively, thus indicating a predominance of heterotrophy (P :R &lt; 1) in these high‐elevation, open‐canopied systems. Bacteria play a strong role in the trophic dynamics of alpine streams.

Invertebrate drift and physico-chemical variables in the tributaries of the Mutt, a Swiss glacial stream

The study of the invertebrate assemblages in a small glacial catchment of the Swiss Alps (the Mutt) was designed to categorise sites in relation to their water sources and assess the contribution of small headwater tributaries to the biodiversity of such Alpine catchments. Physico-chemical characteristics and macroinvertebrate drift were monitored in seven streams during three contrasting hydrological periods in June. August and September 1997. Physico-chemical characteristics clearly differentiated kryal (glacial) and krenal (groundwater-fed) streams. The glacial waters showed a broader amplitude in environmental variation, depending on altitude, geology and season. Macroinvertebrate drift composition was significantly correlated with the environmental conditions. In kryal streams, where harsh conditions prevailed, diversity and abundance were low, while the krenal streams showed highly diversified and dense macroinvertebrate communities, favoured by physico-chemical constancy. However, the low number of taxa found at the same time in these groundwater-fed tributaries and in the main Mutt stream indicated a low contribution from these small streams to the communities of the main stream. Nevertheless, the habitat heterogeneity generated by different water origins in the tributaries favoured the coexistence of diversified macroinvertebrate communities in a rather small catchment area. Therefore these tributaries played an important part in the biodiversity of the glacial catchment.

Water flow and nitrate transport to a groundwater-fed stream in the Belgian-Dutch chalk region

Human activities, such as high fertilizer applications, groundwater abstractions and land use changes, might have a negative impact on drinking water supply, environment and nature reserves of the chalk region in the Dutch–Belgian boundary area. In this area, the groundwater and surface water systems of the Noor experimental catchment have been monitored intensively and modelled to investigate groundwater flow and the transport of nitrate to groundwater-dominated streams. Groundwater flow towards the riparian area, the springs and Noor brook is strongly controlled by long-term variation of the groundwater recharge. Relatively small, but permanent changes in the recharge have a higher impact on spring flow and stream flow than current groundwater abstraction in this chalk catchment. Land use, groundwater flow patterns and the characteristics of the geological formations result in distinctly different groundwater types and associated nitrate concentrations. High concentrations were found under the plateau (median 55 mg · l−1) and extremely low in the riparian area (median 2 mg · l−1). The springs and thereby the Noor brook are mainly fed by groundwater directly coming from the plateau with a low probability of denitrification. This results in median NOconcentrations which are clearly above the drinking water standard of 50 mg · l−1 (i.e. 68 and 58 mg · l−1). Monitoring and modelling show that the north springs have higher NOconcentrations than the south springs due to different probabilities of denitrification. Nitrate in the groundwater-dominated Noor brook is hardly correlated with the discharge, but the major spring shows a clear upward trend in terms of nitrate concentrations, i.e. an increase by about 30 mg · l−1 from 1980 onwards. A policy of reduced nitrogen application will result in lower nitrate concentrations in the surface water system not earlier than in 10 or 20 years, because of the long travel times of water particles in the unsaturated and saturated zone. The north springs will react earlier than the south springs. Copyright © 1999 John Wiley & Sons, Ltd.