SSI FreshWater Catalogue - What is Conductivity?

SSI FreshWater Catalogue

BACKGROUND READING

Freshwater - What is Conductivity?

Last Update: 2024/02/05

Background reading & resources on aspects of the FWC field chemistry measurements - conductivity

Conductivities associated with a range of freshwater bodies

Conductivity levels and its impact on freshwater aquatic life (DataStream, 2022).

Conductivity variability in rainwater variability -catchment examples illustrating, urban seasonal variability; (Zdeb et al., 2018). Local SSI rainwater conducitivty variability not available (yet).

What is Conductivity?

Electrical conductivity in freshwater systems is the chemistry measurement of the salts and inorganic chemical concentrations in water. The measurement reveals how much water conducts electricity. Purer water (eg distilled water) is a very poor conductor of electricity, but as more minerals and/or other components are dissolved in it, a freshwater's conductivity changes.

Conductivity is reported in a unit called a Siemen, a microSiemen (unit - µS/cm), is one-one millionth of a Siemen.

Variability in conductivity in surface freshwater and in ground freshwater (aka groundwater) can be natural, particularly in natural ecosystems dominated settings. In surface freshwater conductivity is derived from varying particulate matter in rainfall and, in surface water bodies, from organic and derived mineral content (and it can vary!). Variations in rainwater chemistry may play a role in conductivity changes in natural surface freshwater chemistry. However, on a small, island scale, seasonal conductivity variations in surface freshwater bodies are more likely to be related to the impact of freshwater inflows into the surface system, from the ground. This "groundwater" (or baseflow), percolating into the surface water bodies (through all seasons), comes from rocks in the ground (or aquifers). Anthropogenically introduced pollutants can also be responsible for conductivity variations in both surface freshwater and groundwater.

In the absence of rainfall (seasonally!) a groundwater-only baseflow into a creek, a lake, or pond freshwater means that freshwater conductivity is derived largely from minerals in the rock(s) through which surface (rain) water has percolated. Changes in conductivity, in creeks with various groundwater baseflows, is influenced by the type of rock type (aquifers) the groundwater is stored in, by an aquifer's storage and water movement characteristics (freshwater circulation capacity, or permeability and soak, or residency, times) within the rocks - think terroir in vineyard (wine/flavour) terminology.

Conductivity variations can also be generated by human activities, such as nitrate fertiliser leached from agricultural soil, pharmaceuticals etc.

Marine seawater is very saline and has high electrical conductivity. The difference in conductivity between seawater and fresh surface or fresh (ground) water are marked, allowing electrical conductivity to be used as an indicator of seawater contamination (or saline intrusion) in coastal or coastally influenced settings. Seawater is very saline and has high electrical conductivity.

Note: for our freshwater data collectors, there is no direct relationship between pH and conductivity. pH is related to the number of hydrogen ions per molecule and conductivity depends on free electrons!

FreshWater Catalogue Creek Chemistry - How do we measure Conductivity?

Our typical FWC freshwater conductivity sampling work is done using compact field measuring probe (or the Oakton). This compact field device is used for spot freshwater (creek, lake, well) measurements in shallow freshwater locations only.

For deeper (lake) conductivity profiling a YSR Pro30 Conductivity logger is used, and full 24/7 creek conductivity data logging is done via a Hobo U24 data logger.

Island FreshWater - Chemistry (Conductivities) & The Hydrological Cycle

The rainwater and groundwater chemistry components of our island's freshwater are likely driven by our island's hydrological cycle, and are entirely dependant on our island's rainfall!

There is a seasonal, year-by-year variability within Spring Island FreshWater Cataloguing surface (largely creek) freshwater chemistry (conductivity) recordings. Conductivity variations happen within (and across) the island's watersheds, giving unique conductivity "fingerprints" to discrete creek or creek systems, these fingerprints (or creek signatures?) are the result of an interplay (interactions) between rainwater and groundwater freshwater chemistries (cf Environment Canada).

The hydrological cycle (courtesy of Dumont & MVIHES, 2017).

Island Freshwater - Conductivities & Creek Flows

Assuming that there is little or no anthropogenic contamination within the island's creek systems, varying conductivities are likely to be driven by seasonal interactions between rainwater (direct precipitation), and other creek inflows (so-called baseflow, derived from ground water). The chemical and temperature characteristics of a creek's ground water inflow are influenced by the natural rainwater infiltration mechanisms - the nature of the soils and rocks through which the rain water flows (vadose zones, aquifers), and the time this infiltration takes, in a process known as groundwater recharge.

Creek systems that do not flow year-round (eg ephemeral or intermittent creeks) may give anecdotal indications of local groundwater levels (water tables) in an area, and provide insights into the ability for rainwater to infiltrate and be stored (influenced by the type of surface cover... and local precipitation levels).

The chemistry of creeks in close proximity to, and downstream of standing bodies of surface water (lakes, ponds), may also influenced by the chemical characteristics of this upstream, "static" (bathtub-like) freshwater volume.

Ephemeral, intermittent and perennial streams (Zaimes & Emanuel), Water flow pathways (Dumont & MVIHES, 2017).

Island Freshwater - Surface vs Ground Water Conductivity Variability

In our island creeks that flow year-round, conductivity variability is attributed to the mixing of a (summer) in-creek "groundwater concentrate" (a groundwater baseflow "cordial") and (winter) rainwater precipitation (cordial "dilutant"). Seasonal (spring, fall) shoulder season chemistry recordings capture the transition from summer to winter creek chemistries - natural creek chemistry variability (fingerprint) phenomena that can be seen across the island!

In selected island wells, subsurface freshwater (groundwater) sampling, captures significantly higher conductivities than typical surface water bodies (creeks, ponds or lakes). The conductivity variability in well groundwater samples relates to several factors: the mineral contents in the groundwater (again the a result of soils and/or rock(s) types through which surface water percolates into the ground), by the freshwater's storage (soak or residency) time within the rock(s) (aquifers), and by potential interactions with (or isolation from) nearer surface freshwater movement/percolation related to rainfall.

The substances dissolved in our island's groundwater are considered to be predominantly natural, such as calcium carbonate dissolved from carbonate-rich minerals. Groundwater components can also be generated by human activities, such as nitrate fertiliser leached from agricultural soil, pharmaceuticals etc., however, these may be largely undiscernible with the current FWC measurement techniques being used. Subtle creek/water body contamination effects would require detailed freshwater cataloguing water quality work to determine if pollution is present (and what types).

Large, static, surface water body chemistries (eg lakes) are often surface (rainwater) dominated, these rainwater dominated (buffered) freshwater settings often complicate basic conductivity (and other) chemistry measurements. In addition to the buffering effect of larger stored rainwater volumes associated with a lake (and with unique, in-lake chemistry variability) , if a lake is supported by multiple inflow creeks this static surface water body may act as a freshwater blender. This mixing environment may overwhelm discrete creek chemistry signatures to the degree that surface vs groundwater components of the lake water body (and lake outflow systems) become diluted and are then difficult to characterise...

While snow buildup and snowmelt are not significant in the SSI world of freshwater, a 2019 "snow cap" spike was visible in our field creek dataset. A snow cap (holding-back seasonal rainfall infiltration) mimics the reduced creek inflow that one normally associates with summer groundwater fed baseflow (and no rainfall) in creeks.

Our FWC field conductivity dataset is now rich enough across parts of the island to split-off local watershed areas, for further studies and/or for expanding local field programmes. We have a number of these "Local Detail" project areas, including one for Fulford Bay. The data gleaned gives insights into surface vs groundwater creek flow contributions and the degree of hydraulic conductivity between the two, a key calibration (ground-truthing) parameter in freshwater system modelling, and water balance analysis.

Cf. our FWC field data interpretation reports and other FWC reporting for further reading on the value of freshwater conductivity measurements.

SSIFWC field data, in-watershed creek/well conductivity comparisons

Island Freshwater - Surface vs Ground Water, What Do We See In Individual Creeks (Watersheds) & Wells Data (Aquifers...)

Island watershed and creek conductivity responses vary across the island, potentially providing local fingerprints of groundwater types and inflow.

For example creek water in the "greater Mereside" area typically has lower values than other island areas - likely a result of the groundwater source rock types and rainwater residence times in these same areas.

Sampled groundwater from our FWC wells shows significant variabilty outside the range of surface creeks, often (though not always) a result of longer freshwater residency times in the subsurface...

Have a look at our "Wells in Watersheds" webpage for more on the evolving creeks vs wells chemistry (conductivity) story.

Island Freshwater Conductivity Data What Does The Variability Telling Us?

For selected island creeks (possibly those more influenced by our ability to measure flow conditions in-stream accurately...?) there are seasonal relationships (a correlation) between flow and conductivity. Where robust these island creek flow-conductivity correlations provide a mechanism to back-out ground water inflow in a creek (cordial - associated with groundwater baseflow) from just surface fresh water (dilutant - from seasonal precipitation). This creek groundwater baseflow volume information can be used to characterise groundwater base flow (and changes therein), as a calibration input to working island hydraulic connectivity and water balance models.

Wells can have higher conductivity values, even above 1,000 µS/cm - likely a function of rock type and “soak time” for the freshwater in contact with the rock, particularly in deeper wells, with indirect and longer freshwater transit times associated with freshwater migration paths. The movement of surface (rainwater) into ground water can take months or years, and some cases even longer.

Our stream sampling is (locally) helping to reveal how much groundwater is leaking out of the ground and into our streams, and perhaps (with a bit more work) well chemistry can be used to give an indicator of groundwater flow-paths, and transit times from rainfall at surface to that well-bore. Hence the interest in the creek and wells chemistry!

How Do Conductivtiy vs Flow Relationships Help Us?

Together with the total flow from a creek system/watershed the (semi-?) quantification of groundwater (baseflow) contributions provides a mechanism to identify volumetrically groundwater inflows (ie discharge into creeks), from the subsurface aquifer systems into watershed systems. If robust this field data plays an important role into calibrating and refining:

island/watershed water budgets
island/watershed hydraulic conductivity assessments

both of these formula input parameters feature in driving robust freshwater models and freshwater sustainability planning.

For more on the developing story of our island's wells have a look at our "Wells In Watersheds" webpage.