Coastal Aquifer Saltwater Intrusion Risk Calculator

Overview: Why Saltwater Intrusion Matters

Coastal aquifers supply drinking water, irrigation, and industrial demand for millions of people. Because they sit next to the ocean, these freshwater bodies are constantly in hydraulic competition with seawater. When conditions are balanced, freshwater flows seaward and keeps denser saltwater pushed back. When pumping, reduced recharge, or sea level rise disturb this balance, saltwater can migrate inland and contaminate wells.

This calculator provides a simple, screening-level estimate of the risk that saltwater will intrude into a coastal aquifer under a given set of conditions. It combines pumping rate, natural or managed recharge, aquifer thickness, distance from the coastline, and sea level rise into a single intrusion risk percentage. The tool is intended for quick exploration and education, not for detailed engineering design.

Conceptual Background

In an unconfined coastal aquifer, fresh groundwater forms a lens that floats on denser seawater. The interface between fresh and saline water is not a sharp line, but for many conceptual calculations it is treated as a boundary whose depth depends on the height of the freshwater table above sea level.

A classic relationship is the Ghyben–Herzberg approximation, which states that, under homogeneous conditions, the depth of the freshwater–saltwater interface below sea level is roughly 40 times the height of the water table above sea level. If pumping lowers the water table by 1 m, the interface can rise by about 40 m. This shows why aggressive pumping near the coast can quickly bring saltwater into the depth range of production wells.

Other key influences include:

• Recharge rate: Freshwater entering the aquifer from rainfall, rivers, or managed recharge basins pushes the interface seaward and counteracts pumping.
• Aquifer thickness: Thicker aquifers provide more vertical separation between wells and the saline interface, allowing more drawdown before salinity becomes a problem.
• Distance from the coast: Wells located farther inland sit above a thicker section of the freshwater lens and are generally 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 does not change.

The calculator translates these influences into a simplified hazard score, which is then converted into an intrusion risk percentage.

Mathematical Formulation

The tool uses an internal dimensionless hazard score, denoted by H. It combines the main drivers with weighting factors:

In plain-text form:

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 emphasizes the effect of over-pumping relative to recharge, while also accounting for geometry (thickness and inland distance) and long-term sea level trends.

Formula in MathML

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

To transform this hazard score into a percentage risk between 0 and 100, 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.

Inputs Explained

Each input field represents a physical aspect of the groundwater system. Choosing realistic values makes the screening-level results more meaningful.

• Pumping rate (m³/day): The total volume of groundwater extracted from the aquifer per day by wells of interest. Larger values increase the likelihood that the freshwater table will be drawn down and the saltwater interface will rise. A small village well might pump a few hundred m³/day, while a large municipal well field could exceed 10,000 m³/day.
• 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 maintained 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, you may use approximate values from regional studies or planning reports for preliminary screening and then refine them as better data become available.

Interpreting the Results

The calculator reports an intrusion risk as a percentage between 0 % and 100 %. This value is not a measured probability, but a relative indicator derived from the weighted inputs. For user-friendly interpretation, the percentage is grouped into qualitative categories:

In practice, the categories should be used as prompts for management discussion rather than strict decision thresholds. For example, a result of 48 % (upper moderate) may warrant actions similar to a result of 52 % (lower high), depending on local priorities and the presence of alternative water sources.

Worked Example

Consider a hypothetical coastal aquifer serving a medium-sized town. Suppose the following conditions:

• 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, such a system is not currently at the highest risk, but it is also not comfortably low. Managers might consider:

• 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 alternative scenarios in the calculator—such as a future sea level rise of 5 mm/yr or an increase in pumping to 7,000 m³/day—can show how sensitive the risk is to management choices and climate pressures.

Comparison of Management Scenarios

The table below illustrates conceptually how different combinations of inputs can shift the intrusion risk. Values are indicative only and will depend on the exact numbers you enter.

By adjusting your inputs to mirror these scenarios, you can explore which levers—reducing pumping, increasing recharge, or relocating wells—have the largest effect on your intrusion risk estimate.

Assumptions and Limitations

This calculator is intentionally simplified. It is best viewed as a screening and educational tool, not as a substitute for site-specific hydrogeologic modeling or professional judgment. Important assumptions and limitations include:

• Homogeneous aquifer: The underlying logic assumes relatively uniform hydraulic properties (permeability, porosity, 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 systems that merit closer study, communicate risks to stakeholders, and compare the relative effect of different management strategies before investing in more detailed analyses.