Saltwater Intrusion Risk Calculator for Coastal Aquifers

Introduction: Coastal aquifer saltwater intrusion basics

Coastal aquifers sit at the boundary between freshwater recharge and seawater pressure, so even modest changes in pumping or rainfall can move the saltwater front inland.

This calculator turns those coastal aquifer stresses into a screening-level saltwater intrusion risk percentage based on pumping rate, recharge, aquifer thickness, distance from the shoreline, and sea level rise. It is meant for quick comparison and education, not for detailed design or regulatory work.

Coastal aquifer intrusion background

In a coastal aquifer, the freshwater lens floats above denser seawater and acts as the main buffer that keeps nearby wells fresh.

The fresh-salt interface is not a perfect line, but screening tools often treat it as a boundary controlled by groundwater head above sea level.

A common coastal aquifer approximation is the Ghyben–Herzberg relationship, which says that under uniform conditions the interface lies about 40 times deeper below sea level than the water table stands above sea level. If pumping lowers the water table by 1 m, the interface can rise by about 40 m. That is why aggressive pumping near the coast can quickly bring saltwater into the depth range of production wells.

Other coastal aquifer controls include:

• Recharge rate: Freshwater entering the aquifer from rainfall, rivers, or managed recharge basins helps push the interface seaward and offset pumping.
• Aquifer thickness: Thicker aquifers give more vertical separation between wells and the saline interface, allowing more drawdown before salinity becomes a problem.
• Distance from the coast: Wells farther inland generally sit above a thicker section of the freshwater lens and are less exposed to intrusion.
• Sea level rise: As sea level rises, the saltwater boundary can move landward and upward, compressing the freshwater lens even if pumping stays the same.

The calculator compresses those coastal aquifer pressures into a hazard score and then maps that score to a percentage risk.

Saltwater intrusion risk formula for coastal aquifers

For this coastal aquifer screen, the hazard score, denoted by H, combines pumping, recharge, aquifer geometry, and sea level rise with fixed weights:

In plain text, the coastal aquifer hazard score is:

H = 0.4 × (Qp / Qr) + 0.2 × (1 / T) + 0.2 × (1 / D) + 0.2 × (S / 5)

where:

• Qp = pumping rate (m³/day)
• Qr = recharge rate (m³/day)
• T = aquifer thickness (m)
• D = distance from the coast (km)
• S = sea level rise rate (mm/yr)

This structure gives the largest weight to pumping relative to recharge, while also acknowledging that thickness, inland distance, and long-term sea level rise influence how easily saltwater advances.

Formula in MathML for the coastal aquifer risk score

The same coastal aquifer hazard score can be expressed using MathML as follows:

To turn that hazard score into a 0-100 risk percentage, the calculator applies a logistic function:

Risk = 100 × 1 / (1 + exp(−(H − 1)))

where exp() is the exponential function. This mapping compresses a wide range of hazard scores into an intuitive 0-100 scale while increasing sensitivity around H ≈ 1.

Coastal aquifer inputs explained

Each input in this coastal aquifer calculator represents a physical control on how far seawater can push inland.

• Pumping rate (m³/day): The total volume of groundwater withdrawn from the coastal aquifer each day. Higher pumping lowers freshwater head and makes intrusion more likely.
• Recharge rate (m³/day): The approximate volume of water entering the aquifer each day from rainfall infiltration, stream losses, or managed recharge projects. When recharge is high relative to pumping, freshwater outflow to the sea is easier to maintain and intrusion risk is lower.
• Aquifer thickness (m): The saturated thickness of the aquifer that contains usable freshwater. Thicker aquifers offer more buffering and can tolerate larger drawdowns before saline water affects wells.
• Distance from coast (km): The horizontal distance between the well field or area of interest and the shoreline. Wells very close to the shoreline are typically more vulnerable to intrusion than those situated several kilometres inland.
• Sea level rise (mm/yr): The long-term rate of mean sea level increase. Higher rates signal stronger pressure from the ocean side over time and a greater tendency for the saline interface to move landward.

If you do not have local measurements, use the best regional estimates you have, then replace them with field data as the project moves from screening toward planning.

Interpreting coastal aquifer saltwater intrusion risk

In this coastal aquifer risk calculator, the percentage is best read as a relative screening index rather than a measured probability of contamination.

In practice, the categories are prompts for management review rather than hard decision cutoffs. A result near a boundary between two bands may deserve the same attention as the band above or below it, especially if local water levels are already trending downward.

Worked example: a stressed coastal aquifer near the shoreline

The coastal aquifer example below shows how moderate pumping and modest sea level rise can turn into a mid-range intrusion risk.

• Pumping rate, Qp = 5,000 m³/day
• Recharge rate, Qr = 6,000 m³/day
• Aquifer thickness, T = 50 m
• Distance from coast, D = 5 km
• Sea level rise, S = 3 mm/yr

First compute the hazard score using the plain-text formula:

1. Compute Qp / Qr = 5000 / 6000 ≈ 0.8333.
2. Compute 1 / T = 1 / 50 = 0.02.
3. Compute 1 / D = 1 / 5 = 0.2.
4. Compute S / 5 = 3 / 5 = 0.6.

Now apply the weights:

• 0.4 × (Qp / Qr) = 0.4 × 0.8333 ≈ 0.3333
• 0.2 × (1 / T) = 0.2 × 0.02 = 0.004
• 0.2 × (1 / D) = 0.2 × 0.2 = 0.04
• 0.2 × (S / 5) = 0.2 × 0.6 = 0.12

Add these contributions:

H ≈ 0.3333 + 0.004 + 0.04 + 0.12 = 0.4973

Next, convert the hazard score into a risk percentage:

1. Subtract 1: H − 1 ≈ 0.4973 − 1 = −0.5027.
2. Negate: −(H − 1) ≈ 0.5027.
3. Compute exp(0.5027) ≈ 1.653 (approximate value).
4. Compute the logistic term: 1 / (1 + 1.653) ≈ 1 / 2.653 ≈ 0.377.
5. Convert to percentage: Risk ≈ 100 × 0.377 = 37.7 %.

Rounded to the nearest whole number, the intrusion risk is approximately 38 %. According to the table above, this falls into the moderate risk category.

In practical terms, this kind of coastal aquifer is not at the highest risk yet, but it is far enough from the low-risk range that managers should keep an eye on salinity trends.

• Tracking long-term trends in groundwater levels and chloride concentrations.
• Developing contingency plans for reducing pumping during drought years.
• Exploring options for enhanced recharge to increase Qr relative to Qp.

Testing an alternative coastal aquifer scenario, such as higher sea level rise, lower recharge, or greater pumping, shows which pressure moves the risk most before you commit to a management change.

Coastal aquifer management scenarios

The table below shows how common coastal aquifer management choices can push the intrusion risk up or down. Values are indicative only and will depend on the exact numbers you enter.

By changing your inputs to mirror these coastal aquifer cases, you can see whether reducing pumping, boosting recharge, or moving wells inland has the biggest effect on intrusion risk.

Assumptions and limitations for this coastal aquifer screening model

This coastal aquifer calculator keeps the physics simple so you can compare scenarios quickly, but that simplicity leaves out many local details. Important assumptions and limitations include:

• Homogeneous aquifer: The underlying logic assumes relatively uniform hydraulic properties such as permeability, porosity, and thickness. Real systems often contain layers, lenses, and faults that can strongly influence intrusion patterns.
• Conceptual steady-state conditions: The formulation does not explicitly simulate transient behaviour such as seasonal recharge pulses, droughts, or short-term pumping surges. It is more consistent with an average or long-term equilibrium.
• Empirical weighting: The numerical weights in the hazard score are approximate and meant to reflect general importance, not calibrated site-specific relationships. In practice, the relative influence of pumping, recharge, and geometry can differ from one aquifer to another.
• Logistic risk scale: The output percentage is a relative index derived from the logistic function, not a measured probability of failure. Two sites with the same percentage may behave differently due to unmodelled factors.
• Limited processes: The tool does not explicitly consider dispersion, density-driven fingering, barrier wells, subsurface dams, or complex boundary conditions such as tidal rivers and estuaries.
• Input uncertainty: Many users may only have approximate values for pumping, recharge, or sea level rise. Uncertainties in these inputs can significantly affect the calculated risk, so results should be interpreted with appropriate caution.

For critical infrastructure, regulatory compliance, or long-term water supply planning, results from this calculator should be supplemented with:

• Field observations of groundwater levels and salinity trends.
• Site-specific numerical or analytical models calibrated to local data.
• Review by qualified hydrogeologists or water resource engineers.

Used appropriately, the calculator can help identify coastal aquifers that merit closer study, communicate risks to stakeholders, and compare the relative effect of different management strategies before investing in more detailed analyses.

How to use this coastal aquifer risk calculator

To use this coastal aquifer risk calculator, enter the best values you have for pumping, recharge, aquifer thickness, distance from the shore, and sea level rise.

1. Enter Pumping Rate (m³/day) using the average withdrawal from the coastal aquifer.
2. Enter Recharge Rate (m³/day) using the recharge that actually reaches the aquifer.
3. Enter Aquifer Thickness (m) for the freshwater-bearing saturated zone you are screening.
4. Run the calculation, then try a second coastal aquifer scenario, such as lower pumping or higher recharge, to see how sensitive the risk estimate is before you make a decision.