Autonomous long-term gamma-spectrometric monitoring of submarine groundwater discharge trends in Hawaii

Effect of atmospheric temperature on underground radon: A laboratory experiment

The effect of atmospheric temperature on underground radon flow was investigated in a customized climate-controlled laboratory (CCL) system, which enabled us to isolate the impact of ambient atmospheric temperature variations on underground radon transport. The soil thermal gradients that developed, following atmospheric warming, acted as the driving force for the diffusive radon flow, resulting in a decrease in the radon concentration along the experimental column setup at a rate of ∼70 Bq∙m^-3 per ^oC∙m^-1 (∼0.4% of the radon concentration). When the ambient temperature decreased, compared to the soil temperature, an air-soil temperature difference developed along the column, which acted as a driving force for radon to flow along the column and promptly increased the radon concentration at a rate of ∼140 Bq∙m^-3 per ^oC (∼0.8% of the radon concentration). The overall radon concentration changes under the experimental conditions were up to 30%. The changes in the molecular diffusion coefficient in the experimental temperature range were ∼7%, with thermal diffusion as a possible additional mechanism contributing to radon transport due to temperature. The cyclic changes in ambient temperature in the forced conditions experiments were found to be directly correlated with underground radon oscillations. The same frequency for ambient temperature and radon concentration, along the experimental column in low frequency warming-cooling cycles (i.e., 4-8 days), was found. This good correlation was lost at higher frequencies (two days or more), due to the asymmetrical response of radon to atmospheric warming and cooling. The results of this study explain some of the field observations in underground radon monitoring.

Traditional and novel time-series approaches reveal submarine groundwater discharge dynamics under baseline and extreme event conditions

Groundwater is a vital resource for humans and groundwater dependent ecosystems. Coastal aquifers and submarine groundwater discharge (SGD), both influenced by terrestrial and marine forces, are increasingly affected by climate variations and sea-level rise. Despite this, coastal groundwater resources and discharge are frequently poorly constrained, limiting our understanding of aquifer responses to external forces. We apply traditional and novel time-series approaches using an SGD dataset of previously unpublished resolution and duration, to analyze the dependencies between precipitation, groundwater level, and SGD at a model site (Kīholo Bay, Hawai'i). Our objectives include (1) determining the relative contribution of SGD drivers over tidal and seasonal periods, (2) establishing temporal relationships and thresholds of processes influencing SGD, and (3) evaluating the impacts of anomalous events, such as tropical storms, on SGD. This analysis reveals, for example, that precipitation is only a dominant influence during wet periods, and otherwise tides and waves dictate the dynamics of SGD. It also provides time lags between intense storm events and higher SGD rates, as well as thresholds for precipitation, wave height and tides affecting SGD. Overall, we demonstrate an approach for modeling a hydrological system while elucidating coastal aquifer and SGD response in unprecedented detail.

Marine electrical imaging reveals novel freshwater transport mechanism in Hawai'i

Conventional hydrogeologic framework models used to compute ocean island sustainable yields and aquifer storage neglect the complexity of the nearshore and offshore submarine environment. However, the onshore aquifer at the island of Hawai'i exhibits a notable volumetric discrepancy between high-elevation freshwater recharge and coastal discharge. In this study, we present a novel transport mechanism of freshwater moving from onshore to offshore through a multilayer formation of water-saturated layered basalts with interbedded low-permeability layers of ash/soil. Marine electromagnetic imaging reveals ∼35 km of laterally continuous resistive layers that extend to at least 4 km from west of Hawai'i's coastline, containing about 3.5 km³ of freshened water. We propose that this newly found transport mechanism of fresh groundwater may be the governing mechanism in other volcanic islands. In such a scenario, volcanic islands worldwide can use these renewable offshore reservoirs, considered more resilient to climate change-driven droughts, as new water resources.

A large buoy-based radioactivity monitoring system for gamma-ray emitters in surface seawater

A buoy (shallow water light type) -based in situ gamma-ray spectrometry system with a 7.6 cmØ × 7.6 cm NaI(Tl) detector for remote real-time monitoring of gamma-ray emitting radionuclides in surface seawater is presented. To convert measured count rates to radioactivity, the full energy peak efficiency of the detector for radionuclides in seawater was estimated using Monte Carlo simulation with the MCNP code. The efficiency calibration was validated by comparing the results with a sampling analysis of ⁴⁰K in seawater at the sites where the monitoring systems were deployed. The minimum detectable activity of the system for ¹³⁷Cs, ¹³⁴Cs and ¹³¹I with gamma-ray measurement time is discussed.

Continuous monitoring of multiple submarine springs by means of gamma-ray spectrometry

The measurement of radiotracers is recognized as a major tool for the investigation and characterization of submarine groundwater discharges, while the use of underwater gamma-ray spectrometry has been proved a robust solution for the qualitative and quantitative determination of radionuclides in the aquatic environment. The capability of online continuous monitoring of submarine springs by means of gamma-ray spectrometry for direct estimation of SGD velocity and discharge is presented. The quantification of SGD flux rate is based on radon progenies time-series provided by two spectrometers placed above the seabed and near the water surface respectively, coupled with water level and meteorological data. The proposed methodology has been applied for a 5-month period in a coastal karstic system where multiple submarine springs occur at Anavalos-Kiveri, Greece. The estimated flux rates derived from the measured activities revealed significant SGD temporal variations with the mean discharge of 12 m³ s^-1 being compatible with previous measurements. The advantages and limitations of direct SGD estimation via underwater gamma-ray monitoring are also discussed.

Development of an autonomous station for measurements of artificial gamma activity in surface water bodies

This paper reports on the structure of the autonomous station for monitoring artificial gamma activity in surface water bodies for the purposes of emergency preparedness of the Czech Republic. A simple design based on the NaI(Tl) submersible detector powered by a combined solar and wind source has been employed. Data transfer is provided by a satellite connection. The detection capabilities of the device have been tested for various unfavourable conditions, and the detection limits have been lowered by using the noise adjustment singular value decomposition (NASVD) method. The detection capabilities of the device fulfil the legal requirements for emergency monitoring, and are almost equal to the detection capabilities of other available devices with a more complicated and less versatile structure.

Development and deployment of an autonomous sensor for the in-situ radioactivity measurement in the marine environment

An autonomous sensor for the in-situ radioactivity measurement is developed，based on NaI(Tl) crystal, to detect the activity concentrations of natural and anthropogenic radionuclides in seawater. Its electronics module, integral enclosure and spectrum analysis were presented. The energy, resolution and efficiency were calibrated. Monte Carlo simulation and the minimum detection estimations were realized. The sensor was tested in the water tank and then successfully deployed for the continuous monitoring in the marine environment in Qingdao offshore for the performance test, background measurement and environmental survey. Some results were deduced from the gamma ray spectra and discussed in comparison with those from the laboratory and literatures.

Mobile underwater in situ gamma-ray spectroscopy to localize groundwater emanation from pockmarks in the Eckernförde bay, Germany

Eckernförde Bay in the Baltic Sea is well-known for the pockmarks areas which are located in the centre and off the southern shore-line of the bay emanating groundwater in a non-continuous but episodic way. Mobile underwater in situ gamma-ray spectroscopy is exploited proving that both ²¹⁴Bi and ⁴⁰K are efficient radiotracers for localization of seepage areas whenever either sediment is in mixture with the emanating fluid or resuspension of surface sediment occurs as a side effect of the fluid emanation.

Impact of Submarine Groundwater Discharge on Marine Water Quality and Reef Biota of Maui

Generally unseen and infrequently measured, submarine groundwater discharge (SGD) can transport potentially large loads of nutrients and other land-based contaminants to coastal ecosystems. To examine this linkage we employed algal bioassays, benthic community analysis, and geochemical methods to examine water quality and community parameters of nearshore reefs adjacent to a variety of potential, land-based nutrient sources on Maui. Three common reef algae, Acanthophora spicifera, Hypnea musciformis, and Ulva spp. were collected and/or deployed at six locations with SGD. Algal tissue nitrogen (N) parameters (δ¹⁵N, N %, and C:N) were compared with nutrient and δ¹⁵N-nitrate values of coastal groundwater and nearshore surface water at all locations. Benthic community composition was estimated for ten 10-m transects per location. Reefs adjacent to sugarcane farms had the greatest abundance of macroalgae, low species diversity, and the highest concentrations of N in algal tissues, coastal groundwater, and marine surface waters compared to locations with low anthropogenic impact. Based on δ¹⁵N values of algal tissues, we estimate ca. 0.31 km² of Kahului Bay is impacted by effluent injected underground at the Kahului Wastewater Reclamation Facility (WRF); this region is barren of corals and almost entirely dominated by colonial zoanthids. Significant correlations among parameters of algal tissue N with adjacent surface and coastal groundwater N indicate that these bioassays provided a useful measure of nutrient source and loading. A conceptual model that uses Ulva spp. tissue δ¹⁵N and N % to identify potential N source(s) and relative N loading is proposed for Hawaiʻi. These results indicate that SGD can be a significant transport pathway for land-based nutrients with important biogeochemical and ecological implications in tropical, oceanic islands.