physics

Submarine groundwater discharge

Submarine groundwater discharge is a physics topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Submarine groundwater discharge rather than just read about it. In short: Submarine groundwater discharge (SGD) is a hydrological process which commonly occurs in coastal areas. It is described as submarine inflow of fresh-, and brackish groundwater from land into the sea.

Key takeaways

Submarine groundwater discharge belongs to physics; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Submarine groundwater discharge to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Submarine groundwater discharge from memory before moving on to harder problems.

Reference excerpt

Submarine groundwater discharge (SGD) is a hydrological process which commonly occurs in coastal areas. It is described as submarine inflow of fresh-, and brackish groundwater from land into the sea. Submarine groundwater discharge is controlled by several forcing mechanisms, which cause a hydraulic gradient between land and sea. Considering the different regional settings the discharge occurs either as (1) a focused flow along fractures in karst and rocky areas, (2) a dispersed flow in soft sediments, or (3) a recirculation of seawater within marine sediments. Submarine groundwater discharge plays an important role in coastal biogeochemical processes and hydrological cycles such as the formation of offshore plankton blooms, hydrological cycles, and the release of nutrients, trace elements and gases. It affects coastal ecosystems and has been used as a freshwater resource by some local communities for millennia.

Forcing mechanisms In coastal areas the groundwater and seawater flows are driven by a variety of factors. Both types of water can circulate in marine sediments due to tidal pumping, waves, bottom currents or density driven transport processes. Meteoric freshwaters can discharge along confined and unconfined aquifers into the sea or the oppositional process of seawater intruding into groundwater charged aquifers can take place. The flow of both fresh and sea water is primarily controlled by the hydraulic gradients between land and sea and differences in the densities between both waters and the permeabilities of the sediments. According to Drabbe and Badon-Ghijben (1888) and Herzberg (1901), the thickness of a freshwater lens below sea level (z) corresponds with the thickness of the freshwater level above sea level (h) as: z= ρf/((ρs-ρf))*h With z being the thickness between the saltwater-freshwater interface and the sea level, h being the thickness between the top of the freshwater lens and the sea level, ρf being the density of freshwater and ρs being the density of saltwater. Including the densities of freshwater (ρf = 1.00 g •cm-3) and seawater (ρs = 1.025 g •cm-3) equation (2) simplifies to: z=40*h Together with Darcy's law, the length of a salt wedge from the shoreline into the hinterland can be calculated: L= ((ρs-ρf)Kf m)/(ρf Q) With Kf being the hydraulic conductivity, m the aquifer thickness and Q the discharge rate. Assuming an isotropic aquifer system the length of a salt wedge solely depends on the hydraulic conductivity, the aquifer thickness and is inversely related to the discharge rate. These assumptions are only valid under hydrostatic conditions in the aquifer system. In general the interface between fresh and saline water forms a zone of transition due to diffusion/dispersion or local anisotropy.

Methods The first study about submarine groundwater discharge was done by Sonrel (1868), who speculated on the risk of submarine springs for sailors. However, until the mid-1990s, SGD remained rather unrecognized by the scientific community because it was hard to detect and measure the freshwater discharge. The first elaborated method to study SGD was done by Moore (1996), who used radium-226 as a tracer for groundwater. Since then several methods and instruments have been developed to attempt to detect and quantify discharge rates.

Radium-226 The first study which detected and quantified submarine groundwater discharge on a regional basis was done by Moore (1996) in the South Atlantic Bight off South Carolina. He measured enhanced radium-226 concentrations within the water column near shore and up to about 100 kilometres (62 mi) from the shoreline. Radium-226 is a decay product of thorium-230, which is produced within sediments and supplied by rivers. However, these sources could not explain the high concentrations present in the study area. Moore (1996) hypothesized that submarine groundwater, enriched in radium-226, was responsible for the high concentrations. This hypothesis has been tested numerous times at sites around the world and confirmed at each site.

Seepage meter Lee (1977) designed a seepage meter, which consists of a chamber which is connected to a sampling port and a plastic bag. The chamber is inserted into the sediment and water discharging through the sediments is caught within the plastic bag. The change in volume of water which is caught in the plastic bag over time represents the freshwater flux.

Pore water profiles According to Schlüter et al. (2004) chloride pore water profiles can be used to investigate submarine groundwater discharge. Chloride can be used as a conservative tracer, as it is enriched in seawater and depleted in groundwater. Three different shapes of chloride pore water profiles reflect three different transport modes within marine sediments. A chloride profile showing constant concentrations with depth indicates that no submarine groundwater is present. A chloride profile with a linear decline indicates a diffusive mixing between groundwater and seawater and a concave shaped chloride profile represents an advective admixture of submarine groundwater from below. Stable isotope ratios in the water molecule may also be used to trace and quantify the sources of a submarine groundwater discharge.

See also Wonky hole, freshwater submarine exit points for coral and sediment covered sediment filled old river channels

References

Worked examples

Example 1 — a first encounter with Submarine groundwater discharge

Start with the simplest possible case. Write down what Submarine groundwater discharge claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In physics, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Submarine groundwater discharge before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Submarine groundwater discharge ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Submarine groundwater discharge

In research: Submarine groundwater discharge appears in physics research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Submarine groundwater discharge in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Submarine groundwater discharge is common in secondary-school and first-year university syllabi. It links to neighbouring topics Biogeochemistry, Fresh water, Physical oceanography, so understanding it makes those chapters shorter.
In everyday life: Look for Submarine groundwater discharge outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Submarine groundwater discharge in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Submarine groundwater discharge means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Submarine groundwater discharge out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Submarine groundwater discharge in simple terms?

Submarine groundwater discharge (SGD) is a hydrological process which commonly occurs in coastal areas. It is described as submarine inflow of fresh-, and brackish groundwater from land into the sea.

Why does Submarine groundwater discharge matter?

Because it connects several physics ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Submarine groundwater discharge?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Submarine groundwater discharge.