Framework

Connectivity Assessment Strategy

Comparison highlights the diverse characteristics of the methods available to assess stream-aquifer connectivity. These variations, particularly in terms of differences in spatial and temporal scale can be used to advantage in an overall assessment strategy. Figure 1 outlines such a strategy that fits within the overall conjunctive water management framework. The components of the assessment strategy are data collation, desktop analysis, field survey and site investigations. The understanding of connectivity at different scales both in time and space brought about by this assessment strategy is bundled into the conceptual model developed for the groundwater and surface water systems of the catchment. In turn, this conceptual model can be translated into a predictive model. This process is iterative, as predictive modelling can highlight information gaps which can spur on additional data collection and assessment.

Figure 1: Components of a strategy for investigation and assessment of connectivity

Data Collation

The initial step is to collate existing baseline data useful in characterising the surface water and groundwater systems of the catchment, and their connectivity. This can be time-consuming and resource-intensive, as data requirements are comprehensive and need to be sourced from multiple agencies. Table 1 summarises the main data themes and their common sources, and include:

1. catchment properties such as boundaries and topography, remote sensing imagery;
2. hydrogeology such as existing geological, soils, regolith or hydrogeological mapping, or borehole databases, geophysical surveys;
3. surface water features, including mapping of drainage and waterways;
4. hydrology, including climate data (rainfall, evaporation), stream gauging, groundwater monitoring and water quality databases;
5. ecosystems, such as wetlands mapping, vegetation mapping and rare/endangered species databases; and
6. catchment use and management, such as land use mapping, water infrastructure, water metering and water allocation.

Table 1: Typical catchment hydrology datasets and sources

Date Type	Data Sources
Catchment Properties
Topography	Topographic maps and DEMs
Hydrogeology
Geology and stratigraphy, aquifer extent and thickness, confining units, bedrock configuration. Aquifer properties (eg hydraulic conductivity, storativity, anisotropy)	Geological maps Geological databases Hydrogeological maps Remote sensing State agency groundwater databases Scientific literature (journal and conference papers, student theses) Unpublished reports (eg consultant reports, drilling programs, geophysical surveys)
Surface Water Features
Drainage network, lakes, wetlands, estuaries	Topographic maps Bathymetric maps
Hydrology
Rainfall and evapotranspiration Run-off and Stream flow Groundwater recharge and discharge Stream-aquifer connectivity Water quality (eq salinity, acidity)	Climate databases Stream gauging data Groundwater monitoring databases Water quality databases
Ecosystems
Aquatic ecosystems Wetlands Rare/endangered species Vegetation	Water management and environmental protection agencies Vegetation mapping Scientific papers Unpublished reports (eg environmental impact statements)
Catchment Use and Management
Land use Water infrastructure (dams, channels, irrigation, extraction bores, flood mitigation and drainage works, interception or injection schemes) Water allocation and use Community requirements and expectations Legal, regulatory and policy setting	Topographic maps Land use mapping Remote sensing State agency databases Catchment authorities (eg CMAs, councils, water authorities) Unpublished reports

Without adequate datasets, analysis of stream-aquifer connectivity cannot be carried out with any degree of confidence. Data needs both spatial and temporal distribution to allow proper interpretation of catchment hydrogeology and hydrology. The quality of data available varies between catchments, and this will affect the type and accuracy of analysis that can be done. The data required to develop a conceptual understanding of connectivity should be determined as early as possible in the planning process. All data sets should meet agreed quality criteria relating to accuracy and temporal and spatial variability.

Further Information

Catchment datasets and sources

Desktop Analysis

The collation of existing datasets provides an opportunity to undertake an initial desktop analysis of connectivity. This can provide preliminary insights into seepage flux without any additional investment in data gathering. Depending on budget and time constraints, this may be the extent to which an assessment can be made. A desktop analysis can include the approaches of:

1. hydrogeological mapping, where available data such as borehole information, pump tests, groundwater monitoring, geophysics and geology or soils mapping are combined to compile maps such as groundwater potentials, flow directions, salinity, aquifer structural contours and hydraulic conductivity distribution. These are necessary in providing the hydrogeological setting for the stream;
2. hydrographic analysis, where various methods can be applied to the available time-series of stream flow to characterise the baseflow component;
3. water balance, where the stream flow record from multiple stream flow gauges can be used to derive the water balance for the intervening stream reaches, or for the overall catchment;
4. hydrochemistry, where any existing water quality monitoring (such as EC, pH, major ions, nutrients) is used to define variations in surface water and groundwater chemistry that reflects changes in seepage flux in time and space;
5. geophysics and remote sensing, where available imagery and data can be processed to provide information on catchment parameters that either control or indicate connectivity;
6. hydrometrics, most notably when the available time series of stream water levels are compared with nearby monitoring of watertable elevation, to determine changes in the potential direction of groundwater flow. Historic pump tests can indicate the magnitude of aquifer transmissivity; and
7. temperature monitoring, acknowledging that stream temperature is commonly monitored (in conjunction with stream level and also for ecological purposes) and can be potentially also be used in assessing seepage flux.

Field Survey

Additional field surveys can be undertaken to support the initial desktop analysis. These surveys are used to provide greater spatial resolution by interpolating along the stream between existing monitoring sites and to infill areas in the catchment with insufficient data. Such surveys are also used to identify key sites that require more intensive investigations. Initial conceptualisation of processes can also be verified. Examples of these survey methods include:

1. hydrochemistry, where water samples are taken along the stream network and analysed for environmental tracers. This is commonly done to highlight trends and hotspots for groundwater discharge to streams. The tracers analysed range from the simple and cheap (such as field EC and pH) to the more sophisticated and expensive (such as stable isotopes and radon);
2. geophysics, where surveys can be undertaken down the length of the stream or across the extent of the catchment. For example, geo-electric arrays can be towed behind boats to map the electrical conductivity of the water column and underlying sediments. The technique has been particularly effective in mapping seepage of highly saline groundwater. Ground-based seismic traverses can be useful in mapping geological features that constrain or control groundwater flow, such as the geometry and stratigraphy of alluvial aquifers or fault zones;
3. water balance, where flow is measured at multiple points along the stream to help target hotspots in terms of gross seepage losses or gains. Simple water balances can be estimated relatively quickly and cheaply to derive an initial rough estimate of the direction and magnitude of seepage on a stream reach basis;
4. ecological indicators, involving the reconnaissance survey of indicator species such as specific aquatic plants, phreatophytes or hyporheic biota to map groundwater discharge hotspots. Other field observations (such as precipitates, water colour) can be included in the reconnaissance; and
5. temperature measurements, used as a screening tool to identify gaining and losing stream reaches. This is particularly useful if the groundwater discharge has a significantly higher temperature than the ambient stream, either due to geothermal conditions or due to release from deep regional groundwater flow systems.

Site Investigations

More intensive investigations can be undertaken at key sites in the catchment. This is commonly undertaken to confirm key processes, quantify seepage fluxes or provide more information relating to the hydrological, chemical or ecological aspects of connectivity. Such sites are selected on the basis of the initial desktop analysis and field surveys. Investigations at these specific sites can include:

1. seepage measurement, involving the installation of seepage meters to estimate the direction and magnitude of seepage flux. As it is a direct measurement, seepage meters have the potential to validate indirect methods that involve measuring secondary indicators such as hydraulic head difference, chemical tracers or isotopes;
2. hydrometric analysis, where piezometers and stream gauges are installed to allow local comparison of groundwater and surface water levels and so define hydraulic gradients. Pump tests can be undertaken to estimate shallow aquifer transmissivity;
3. artificial tracers, by running a tracer test to quantify aquifer parameters and fluid transport properties, particularly in highly variable aquifers (such as fractured rock or karsts) and in solute transport studies (such as contaminants and nutrients). Specific tracers can be used to track pollutants such as human pathogens, where the movement and fate of these pollutants may not match water flow. Tracers can be used to assess the significance of local geological features (such as faults, clay layers or cave systems) on stream-aquifer connectivity; and
4. temperature studies, with time-series monitoring of temperature fluctuations for the stream and the sediment profile at varying depths to evaluate seepage flux and hydraulic conductivity. Temperature loggers are robust, simple and relatively inexpensive and available for various scales of measurement.