Tropical Beaches Attenuate Groundwater Nitrogen Pollution Flowing to the Ocean.

Tropical urbanized coastal regions are hotspots for the discharge of nutrient-enriched groundwater, which can affect sensitive coastal ecosystems. Here, we investigated how a beach modifies groundwater nutrient loads in southern India (Varkala Beach), using flux measurements and stable isotopes. Fresh groundwater was highly enriched in NO3 from sewage or manure. Submarine groundwater discharge and nearshore groundwater discharge were equally important contributors to coastal NO3 fluxes with 303 mmol NO3 m-1 day-1 in submarine and 334 mmol NO3 m-1 day-1 in nearshore groundwater discharge. However, N/P ratios in nearshore groundwater discharge were up to 3 orders of magnitude greater than that in submarine groundwater, which can promote harmful algae blooms. As groundwater flowed through the beach, N/P ratios decreased toward Redfield ratios due to the removal of 30-50% of NO3 due to denitrification and production of PO4 due to mineralization of organic matter. Overall, tropical beaches can be important natural biogeochemical reactors that attenuate nitrogen pollution and modify N/P ratios in submarine groundwater discharge.

A seasonal 222Rn mass-balance of Lake Burullus, Egypt: Indications for higher pore water exchange rates during the dry season.

Radon mass balances in lakes can be used to trace transport processes along the sediment-water interface, such as groundwater discharge or pore water exchange. Understanding these transport processes is important, as they can affect the lake water budget, or biogeochemical cycles in lakes due to nutrient inputs. We present here a seasonal 222Rn mass balance of Lake Burullus (Northern Egypt), the second largest lake of Egypt. The Lake receives 222Rn from drainage water and the decay of 226Ra, and loses 222Rn via decay of 222Rn, atmospheric evasion and water discharge into the Mediterranean Sea. However, the mass balance reveals a significant surplus of 222Rn, which cannot be explained by the previously mentioned processes. The 222Rn surplus is especially high during the dry season, and might be explained by groundwater discharge and pore water exchange which transports 222Rn into the lake. Higher 222Rn fluxes into the lake during dry season might be explained by higher bioirrigation rates due to warmer temperatures, or higher groundwater discharge rates due to a higher hydraulic head on land caused by excessive irrigation in adjacent fields. Groundwater discharge and pore water exchange should be investigated in further detail in order to understand if they have effects on water budgets and biogeochemical cycles of Lake Burullus.

Microbial community structure associated with submarine groundwater discharge in northern Java (Indonesia).

Submarine groundwater discharge (SGD) can be an important pathway for chemical or biological pollutants from land to the ocean around the world. However, studies on the microbial communities associated with SGD in Southeast Asia, which has been hypothesized as SGD hotspot, remain scarce. In this study, we examined the microbial community composition with 16S rRNA gene sequencing along the hydrological continuum of an SGD site in a tropical urban area of Indonesia. Of the observed parameters in this study, salinity and temperature were the most determinant variables explaining patterns in microbial community composition. The bacterial taxon Burkholderiaceae was predominantly found in low salinity samples, including those from terrestrial groundwater and brackish pore water, while cyanobacteria of the genus Synechococcus sp. CC9902 were indicative of saline SGD and seawater samples. The composition of microbial taxa in each sample pointed to the influence of shallow terrestrial groundwater in the beach pore water, while seawater recirculation dominated the SGD sampling points situated further offshore. We identified taxa containing fecal indicators and potential pathogens at the SGD compartments; however, while a likely explanation, we could not conclude with certainty that SGD was a conduit for these bacteria. Overall, the results from this study show that microbial community analysis can highlight hydrological processes and water quality at the SGD site; thus, they could be useful for environmental policymakers to formulate water management strategies in coastal areas.

Groundwater nutrient inputs into an urbanized tropical estuary system in Indonesia.

Groundwater discharge is known to transport nutrients into estuaries at several locations around the world. However, few studies report groundwater-associated nutrient fluxes from tropical developing regions such as Southeast Asia, even though this area shows the strongest human modifications in the coastal zone worldwide. We investigated groundwater nutrient flux into two streams and estuaries (Awur and Sekumbu Bay) in the urban area of Jepara, Indonesia, and its relation with the land usage surrounding the estuaries. We found that average concentrations of NO3, NH4, and PO4 in Jepara's aquifer reached 145µM, 68µM, and 14µM, respectively, and our results indicate that these were mainly originated from untreated sewage, agriculture, and manure input. Approximately 2200tonNyear-1 and 380tonPyear-1 were removed in the soil and aquifer before the nutrients were discharged into the river. The total groundwater discharge into the river and estuary was estimated to 461×103m3d-1, or up to 42% of the river discharge. Discharge of groundwater-associated NO3 (72×103mold-1), NH4 (34×103mold-1), PO4 (5×103mold-1), and additional surface runoff may contribute to eutrophication and a decrease of nearshore surface water quality. Nutrient concentrations in groundwater, river, and coastal seawater in the Jepara region are similar to those found in major urban areas in Southeast Asia, e.g. Manila and Bangkok, even though Jepara has smaller size and population. Thus, our results indicate that medium populated cities with highly modified regional land use can contribute a significant amount of nutrient discharge in the coastal area and should be included in global assessments of nutrient budget calculation.