Submarine groundwater discharge and its ecological influences revealed by coupling radon-222, thermal remote sensing of satellite and unmanned aerial vehicles model

Abstract

Submarine groundwater discharge (SGD) is the mixture of fresh groundwater driven by terrestrial recharge and recirculated seawater constrained by marine forces and can import vast nutrient loadings to the ocean. An effective approach of SGD quantification is advantageous to probe the ecological influences of SGD on coastal environments. However, current approaches based on physical, chemical, and geotracer means can only lead to a limited sparse-point monitoring due to the time-consuming and laborious disadvantages. Hence, this thesis developed an approach based on the in-situ radon-222 (222Rn) and thermal infrared remote sensing of satellites and emerging unmanned aerial vehicles (UAV) to efficiently quantify SGD and depict its variations from daily, monthly to yearly, thus greatly improving the temporal resolution of SGD monitoring. This approach was successfully validated against an entire-year field data for UAV and a time-span of two decades for satellites.

With a refined SGD quantification, this thesis investigated two severe coastal environmental problems, algal blooms and fecal contamination, both potentially associated with SGD. Algal blooms, denoted as the rapid growth of microscopic phytoplankton, are frequently observed worldwide. For attesting to the grounded effects of SGD on the mechanism regulating the formation of algal blooms, data collected over one year and retrieved from past three decades including direct observations of algal blooms, coastal and groundwater nutrients and process model output of water vertical stability were analysed and demonstrated that the physical control (via water vertical stability) and chemical control (via phosphate of SGD loadings) jointly formatted algal blooms.

This thesis was also motivated to improve the mechanistic understanding of the microbial occurrence and transport in the beach groundwater - surf zone system for a better prevention and mitigation of coastal fecal contamination. By conducting one-year field monitoring of the commonly used fecal indicator bacteria, Escherichia coli (E. coli), in the beach groundwater - surf zone system, this thesis for the first time proposed that 222Rn is an effective analogue of E. coli in this system. The robust and unique relationship between 222Rn and E. coli can provide additional critical context to microbial water quality assessments. Also, this thesis investigated the mechanism of stormwater disturbing the E. coli dynamics in the beach aquifer by two time-series field observations during the periods with storms and without storms. Results illustrated that net E. coli growth was both enhanced in groundwater and seawater during storm periods, ascribed to the storm-induced intense rainfall and beach sand erosion. The finding was constructive to the understanding of beach ecosystem affected by storm events and highlighted the ecological controls of SGD on surface water quality. Finally, based on the findings that UAV thermal images can physically reflect 222Rn activity, and the robust relationship between 222Rn and E. coli, this thesis established an E. coli predictive model which was validated and tested against the in-situ data from 50 beaches. This model highlighted that the UAV technology can be promising as a complement of traditional E. coli monitoring, and further pave the way for effective public health risk warning.