Investigate and Assess
Traditionally, surface water and groundwater resources have tended to be independently assessed. In taking a conjunctive water management approach, the characteristics of surface water features and groundwater systems in a catchment are both still investigated. The important addition is that the interactions between surface water features and the groundwater systems are also assessed. The nature and level of the assessment will depend on:
- the management issues within the catchment, such as salinity or over-extraction of water;
- the significance of the water resource in terms of social, economic and environmental values;
- the relative development of the water resource in terms of the ratio between use (or allocation) and sustainable limits;
- any risk assessment of the likely magnitude of impacts associated with the management issue, such as loss of economic productivity, land and water degradation or poor ecosystem health;
- the availability of resources such as data, budget and expertise; and
- the management and policy timeframes.
Hence, the assessment associated with a conjunctive water management approach includes investigation of:
- surface water features including streams, dams, wetlands and estuaries. This includes such aspects as flow duration and dynamics, water storage capacity, water quality, aquatic ecosystems, land use impacts, climate variability and water extraction regimes;
- groundwater systems. This covers aspects such as aquifer geometry, geological and stratigraphic configurations, hydraulic properties such as transmissivity and storativity, water sources and sinks such as recharge, abstractions and discharge mechanisms, environmental dependencies and the impacts of land use; and
- surface water-groundwater interactions, involving the analysis of the dynamics of water flow between aquifers and surface water features, and the impacts of this interaction in terms of water quantity, quality and ecology (Figure 1).
Figure 1: Coordinated assessment of surface water and groundwater systems
The focus of assessment is to acquire the baseline information to describe the characteristics of surface water and groundwater systems of the catchment, and their interactions, both spatially and temporally. This can include:
- collation and interpretation of existing catchment datasets that can be used to describe the hydrological and hydrogeological attributes. Catchment-wide datasets such as climate parameters (rainfall, evaporation), topography, surface drainage, geology/geomorphology and land use need to be collected in the first instance;
- collation and interpretation of existing monitoring that can describe the spatial and temporal variability of groundwater and surface water systems. The key databases are the available time series record of water levels, flow and quality parameters; and
- identification of key information gaps and the initiation of specific studies to clarify key processes.
The focus of this assessment is to acquire the baseline information to describe the characteristics of surface water and groundwater systems of the catchment, and their interactions, both spatially and temporally.
A wide range of tools is available to assess the nature and degree of the interaction between surface water and groundwater systems. These include:
Field Observations
Visual indications of seepage flux can be observed in certain catchments and settings. An initial reconnaissance can highlight hotspots where groundwater is discharging to streams, helping define useful parameters to measure and to identify management issues that are impacted by connectivity. Examples of field indicators include direct observation of water flow from springs at the margins or within the stream bed, water vapour or ice-free conditions around springs during winter, mineral precipitates or iron-bacteria accumulations, or changes in water colour or odour.
Seepage Measurement
The direct measurement of seepage flux at the stream-aquifer interface can be undertaken using seepage meters and similar devices. The basic concept is to cover and isolate the stream bed with an inverted open chamber and measure the change in volume of water contained in a bag attached to the chamber over a measured time interval. Additional water in the bag over the time of operation indicates gaining stream conditions. Several modifications have been made to the design and operation of the seepage meter to address potential sources of measurement error and to handle logistical issues. Automated versions using different technologies have been developed to enable real-time monitoring of seepage flux.
Ecological Indicators
Specific vegetation communities or biota can indicate groundwater discharge to surface water features. Changes in the composition and accumulated biomass of submerged aquatic plants can relate to groundwater seepage. The near-stream presence of phreatophytic plants, which are deep-rooted and can access groundwater, can indicate a shallow watertable. The extent and composition of biota that habitat the hyphoreic zone, can also describe the processes of near-stream groundwater and surface water mixing.
Hydrogeological Mapping
Knowledge of the hydrogeology surrounding a surface water feature is critical in understanding connectivity. This involves mapping the configuration and characteristics of the groundwater flow systems within the catchment. This covers aspects such as aquifer geometry, host geology and stratigraphy and hydraulic properties (such as transmissivity and storativity). Also included are specific geological features such as faults, facies changes or river geomorphology that can locally control groundwater flow.
Geophysics and Remote Sensing
Geophysical and remote sensing technologies such as airborne electromagnetics (AEM), radiometrics, seismic waves, electrical charge, or satellite imagery can be used to interpret connectivity. These surveys can map the variation in parameters such as groundwater salinity, vegetation types or soil moisture that can be secondary indicators of groundwater discharge. They can also be used to identify geological features that control seepage flux. Mapping of landscape parameters (such as soil type, land use, vegetation cover) that can have an impact on seepage flux can also be supported by geophysical or remote sensing technologies.
Hydrographic Analysis
The stream hydrograph can be processed and analysed to characterise the magnitude and timing of groundwater discharge to streams. Baseflow separation techniques use the time-series record of stream flow to derive a baseflow hydrograph. Of these, recursive filters are the most commonly applied. Frequency analysis takes a different approach by deriving the relationship between the magnitude and frequency of stream flows. Recession analysis focuses on recession curves which follow flow peaks. These curves are fitted using storage-outflow models to characterise the natural storages that feed the stream.Hydrometric Analysis
Hydrometric methods are based on Darcy's Law, so focus on the hydraulic gradient between groundwater and surface water systems and the hydraulic conductivity of the intervening aquifer and bed material. Piezometers are used to measure groundwater levels which are compared with the elevation of the stream stage. Pump (or slug) tests can be undertaken on these piezometers to estimate the transmissivity of the aquifer material.
Hydrochemical Studies
Interpretation of the chemical constituents of water can provide insights into stream-aquifer connectivity. Dissolved constituents can be used as environmental tracers to track the movement of water. For example, a particular characteristic of the groundwater chemistry (such as high radon levels) can be used as an indicator of groundwater discharge when measured in the surface water. Environmental tracers can occur naturally or can be released into the general landscape by human activities. Some of the commonly used environmental tracers include field parameters such as EC or pH; the major anions and cations such as calcium, magnesium, sodium, chloride and bicarbonate; stable isotopes in the water molecule of oxygen-18 (18O) and deuterium (2H); radioactive isotopes such as tritium (3H) and radon (222Rn); and industrial chemicals such as chlorofluorocarbons (CFC) and sulphur hexafluoride (SF6).
Temperature Studies
Heat can also be used as a tracer to characterise seepage flux. Time series monitoring of temperature in both the surface water and groundwater systems is used. Stream temperatures have a characteristic diurnal pattern overprinting seasonal trends, whilst regional groundwater temperatures tend to be relatively constant at the daily scale. Temperature monitoring at varying depths in the stream bed can indicate the relative influence of groundwater and surface water processes. Numerical models of heat flow (such as VS2DH and SUTRA) can be used to quantify seepage flux.
Artificial Tracers
Artificial tracer tests are used to evaluate the extent to which aquifers interact with streams, providing information on groundwater flow paths, travel times, velocities, dispersion, flow rates and the degree of hydraulic connection. These tests involve the introduction of a tracer material or chemical and subsequent monitoring of its movement. This differs from environmental tracer methods which rely on the measurement and interpretation of background concentrations. Fluorescent dyes (such as Rhodamine WT), conservative major ions (such as chloride or bromide), organic compounds (such as ethanol or fluorinated benzoates), isotopes (such as selenate or deuterium), non-pathogenic microorganisms or colloidal material (such as clubmoss spores) have been used in tracer studies.
Water Budgets
A common approach to investigating seepage flux between a stream and underlying aquifer is to measure stream flow at specific points. These measurement sites subdivide the stream into reaches and a water budget is estimated for each reach, accounting for inputs such as tributary flows and outputs such as evaporative losses and diversions. The difference between inflows and outflows is then attributed to the seepage flux. The method relies on accurate measurement of stream flow and appropriate accounting of the other gains and losses.
Relevant Links
Environment Canterbury NZ Guidelines for the assessment of groundwater extraction effects on stream flow
US Geological Survey Recent advances in understanding the interaction of groundwater and surface water
US Geological Survey Hydrologic and chemical interactions between surface water and ground water
UK Environment Agency High-resolution in-situ monitoring of flow between aquifers and surface waters
UK Environment Agency Understanding geochemical fluxes between groundwater and surface water