Case study · Field hydrogeology · MSc thesis

Where do Guatemala’s coastal mangroves get their freshwater?

The Retalhuleu district holds the largest mangrove area on Guatemala’s southern Pacific coast (5,111 ha) — a groundwater-dependent ecosystem where fresh groundwater discharge regulates salinity levels. The same aquifer is the main source of drinking water and irrigation for a booming agricultural region, yet almost no hydrogeological information existed. Mangrove area experience a long dry season, and therefore use groundwater.

What we did

A dry-season field campaign across the Samalá alluvial aquifer: inventorying wells, measuring groundwater heads, and sampling for hydrochemistry and stable isotopes (δ¹⁸O, δ²H). Three independent lines of evidence were combined into one conceptual flow model: water chemistry evolving from Ca-HCO₃ inland to Na-HCO₃ at the coast – evidence of long flow paths through carbonate diss. and cation exchange; isotope signatures at the coast too light to be local rain, matching precipitation that fell high on the volcanic arc; and a water balance with recharge estimation to close the story.

Stable isotope ratios of groundwater samples plotted against distance to the coast: samples near the coast carry the depleted signature of highland rain rather than local coastal precipitation — The isotope evidence: δ²H and δ¹⁸O of groundwater versus distance to the coast, against the local rain stations (dashed lines). Coastal discharge waters are isotopically lighter than coastal rain — they fell as precipitation high in the volcanic arc.

What we found

The water discharging into the mangrove forests is deep, long-traveled groundwater recharged on the slopes of the volcanic highlands, tens of kilometers upstream — not local rainfall. That single finding reshapes how the system must be managed: protecting the mangroves means protecting recharge areas far inland, and sustainability depends on long-term average recharge rather than local droughts. Because the water travels for years, even strong El Niño events barely touch the groundwater system that keeps the mangroves alive. Groundwater heads near the coast already dip below sea level, so the balance is delicate: excessive pumping risks saltwater intrusion and would shift the salinity gradient that sets the natural zonation of mangrove species.

Conceptual hydrogeological cross-section from the volcanic highlands (75 km inland) to the coast: groundwater recharges in the Tertiary and recent volcanics, flows through the Samalá and Pleistocene aquifers, and discharges at the mangroves next to the saltwater wedge — The conceptual model: rain recharging the volcanic highlands travels ~75 km through the Samalá and Pleistocene aquifers and discharges at the mangroves (GDEs), holding back the saltwater wedge.

Cantarella, V. P. (2022). Hydrogeological investigation in coastal mangrove ecosystems of the Pacific coast of Guatemala. MSc thesis, GroundwatCH (IHE Delft · IST Lisboa · TU Dresden), hosted by ICC Guatemala.