Drip Water Monitoring Research Articles

An underground drip water monitoring network to characterize rainfall recharge of groundwater at different geologies, environments, and climates across Australia

Abstract. Understanding when and why groundwater recharge occurs is of fundamental importance for the sustainable use of this essential freshwater resource for humans and ecosystems. However, accurately capturing this component of the water balance is widely acknowledged to be a major challenge. Direct physical measurements identifying when groundwater recharge is occurring are possible by utilizing a sensor network of hydrological loggers deployed in underground spaces located in the vadose zone. Through measurements of water percolating into these spaces from above, we can record the potential groundwater recharge process in action. By using automated sensors, it is possible to precisely determine when recharge occurs (which event, month, or season and for which climate condition). Combined with daily rainfall data, it is possible to quantify the “rainfall recharge threshold”, the amount of rainfall needed to generate groundwater recharge, and its temporal and spatial variability. Australia's National Groundwater Recharge Observing System (NGROS) provides the first dedicated sensor network for observing groundwater recharge at an event scale across a wide range of geologies, environments, and climate types representing a wide range of Australian hydroclimates. Utilizing tunnels, mines, caves, and other subsurface spaces located in the vadose zone, the sensors effectively record “deep drainage”, water that can move beyond the shallow subsurface and root zone to generate groundwater recharge. The NGROS has the temporal resolution to capture individual recharge events, with multiple sensors deployed at each site to constrain the heterogeneity of recharge between different flow paths, and to quantify (including uncertainty bounds) rainfall recharge thresholds. Established in 2022, the network is described here together with examples of data being generated.

Characteristics and Influencing Factors of Dissolved Organic Matter in Cave Drip Water—A Case Study from Furong Cave, Southwest China

Understanding the hydrological processes of dissolved organic matter (DOM) in the surface karst zone is crucial for the utilization and safety of groundwater resources. However, research on DOM in drip water from karst caves is limited. In this study, continuous monitoring was conducted for four years at four drip water monitoring sites (MP1, MP2, MP3, and MP9) in Furong Cave, Southwest China. The three-dimensional fluorescence excitation–emission spectroscopy (3D-EEM) and fluorescence region integration (FRI) methods were employed, along with correlation analysis, to investigate the spectral characteristics, composition, sources, and influencing factors of the DOM in the drip water of Furong Cave. The results indicated that (1) the three-dimensional fluorescence peaks in the drip water were classified into six categories: A, B, C, T, Ti, and M. (2) The dominant source of the drip water DOM is endogenous organic matter. (3) The FRI analysis indicates a relatively high proportion of Type II substances in the drip water, predominantly composed of tryptophan-like substances. (4) The DOM in the drip water of Furong Cave was influenced by various factors, including the mixing effect of “new” and “old” water, water residence time in karst systems, and water–rock interactions (WRI), resulting in the complex responses of drip rates and DOM to surface precipitation and temperature at different drip water sites. This study provides a reference for comparative research on DOM in cave drip water in karst regions, which contributes to a better understanding of the migration mechanism of DOM in karst aquifers under different climate and karst ecological conditions.

Response of drip water Mg/Ca and Sr/Ca variations in ventilated caves to hydroclimate

Mg/Ca and Sr/Ca in speleothems which record valuable information regarding past variations of precipitation and cave air pCO2 are promising proxies because the degrees of water-rock interaction (WRI) and prior calcite precipitation (PCP) are directly and indirectly related to these changes. However, the controls on Mg/Ca and Sr/Ca can be complex, and most studies ignored the combined effects of rainfall and cave air pCO2. Moreover, knowledge of the influence of seasonal rainfall and cave air pCO2 on seasonal fluctuations in drip water Mg/Ca and Sr/Ca are limited for caves with different regions and ventilation types. Drip water Mg/Ca and Sr/Ca were monitored for five years at Shawan Cave. The results indicate that the irregular seasonal oscillation in drip water Mg/Ca and Sr/Ca is controlled by inverse-phase seasonal changes between rainfall and cave air pCO2. The rainfall amount may be the primary controlling factor of the interannual variation in drip water Mg/Ca, whereas the interannual variation in drip water Sr/Ca is most likely controlled by cave air pCO2. Furthermore, we compared drip water Mg/Ca and Sr/Ca of caves in different regions to fully understand how drip water Mg/Ca and Sr/Ca respond to hydroclimate changes. The drip water element/Ca, for seasonal ventilation caves with a fairly narrow range of cave air pCO2 respond well to the local hydroclimate associated with rainfall variation. If the range of cave air pCO2 is considerably large, the element/Ca in seasonal ventilation caves of subtropical humid regions may not reflect hydroclimate and that of Mediterranean and semi-arid regions may be primarily controlled by cave air pCO2. The element/Ca in the low year-round pCO2 caves could reflect the hydroclimate associated with surface temperature. Therefore, observations of drip water monitoring and comparative analysis can provide a reference for the explanation of speleothems element/Ca ratios from seasonally ventilated caves worldwide.

Soil loss through fissures and its responses to rainfall based on drip water monitoring in karst caves

Owing to the binary geologic structure, soil loss in the karst region is characterized as a combination of surface erosion and underground soil loss. However, we know little about the process and amount of underground soil loss by now due to the difficulty in observation and identification. To predict soil loss accurately and control rocky desertification effectively, it is urgent to collect data of underground soil loss directly by new ways. In this study, 137Cs tracer technique, high frequency online monitoring and UAV survey were used to investigate soil loss through fissures. The soil samples were collected in five karst caves of Guizhou Province for determining 137Cs activity. In two of these caves, the improved monitoring devices were adopted to monitor drip water online, and the sediment content in drip water was measured once a month. Furthermore, depending on 5 exposed karst sections, the distribution density of fissures was surveyed with UAV to calculate the average underground soil loss modulus in the watershed. The results showed that (1) the soil materials deposited in caves were mainly transported by runoff from the cave entrance, rather than through fissures above the caves. (2) The initial response time and duration of drip water in caves to rainfall were controlled by surface land coverage and the process of precipitation. And the variation of sediment content in drip water kept almost constant with rainfall except at the beginning of the rainy season. (3) The directly measured underground soil loss rate monitored through the fissure above a cave with high bedrock exposure rate and a cave with high vegetation coverage was 2.42 g·yr−1 and 0.36 g·yr−1, respectively. (4) The average underground soil loss modulus in the area with high bedrock exposure rate and with high vegetation coverage was 0.35 t·km−2·yr−1 and 0.05 t·km−2·yr−1, respectively. This study is helpful for understanding the process and intensity of underground soil loss.

Hydrogeochemical responses of cave drip water to the local climate in the Liangfeng Cave, Southwest China

Abstract The hydrogeochemistry of cave drip water is an important environmental index in cave systems, and drip water monitoring may be an essential solution for paleoclimate reconstructions. We measured the hydrogeochemical properties of the seasonal and perennial drip water and CO2 concentrations from 2015 to 2019 in the Liangfeng Cave, Guilin, Southwest China. This study identified the difference in the regional environmental records in the perennial and seasonal drip water. By comparing the regional climate data recorded by the drip water, the results showed that the perennial drip water recorded regional climate information throughout the year, whereas the seasonal drip water only recorded the high precipitation periods. The precipitation during the 2015 dry season was abnormally high, and higher than the values in other rainy seasons. This indicates that hydrogeochemistry only represents changes in precipitation and not the alternation of dry and rainy seasons during this period.

Typhoon rainfall impact on drip water δ18O in Xianyun cave, Southeast China

AbstractPrecisely dated high‐resolution speleothems may record past typhoon events, however, the state of the art cave monitoring is a prerequisite to identify suitable stalagmites for the reconstruction of such events. With this motivation, we examined the isotopic composition (δ18O and d‐excess values) of rainfall, outside river, cave drip water, and an underground river in the Xianyun cave system, located in southeastern China. Monthly to bi‐monthly monitoring of environmental and isotopic conditions was conducted for 1 year, from December 2018 to December 2019, including a typhoon event (August 24, 2019 to August 26, 2019), called Bailu. The δ18O of rainfall samples over the cave and outside river water ranged from −9.7‰ to −1.9‰ and −8.2‰ to −6.3‰, respectively, while the δ18O of Typhoon Bailu rainfall and instantaneous outside river water ranged from −19.6‰ to −6.3‰ and −10.4‰ to −7.7‰, respectively. Typhoon Bailu‐induced rainfall showed distinctly negative δ18O values as compared to those of the monthly and bi‐monthly rainfall, exhibiting a three‐stage inverted U‐shaped variation characteristic. Four drip water monitoring sites inside the cave revealed low variations during the studied period with average values of −7.8‰, −8.0‰, −8.0‰, and −8.1‰. However, during the typhoon, the drip water δ18O values exhibited similar characteristic as outside rainfall but with just 0.2‰ negative deviation owing to precipitation amount and drip water source reservoir. The integration of rainfall amount with drip water source reservoir determines the degree to which a typhoon isotopic signature gets diluted during epikarst infiltration. This study provides the first instrumental evidence of typhoon signal in karst system in southeastern China. Our results imply that the δ18O of drip water in Xianyun cave can instantaneously respond to typhoon rainfall. However, the 0.2‰ shift in drip water δ18O is difficult to be recorded by speleothems. We suggest multi‐year monitoring to ascertain fully if the stalagmites could be used as paleotyphoon proxy.

Quantifying temporal variability and spatial heterogeneity in rainfall recharge thresholds in a montane karst environment

Quantifying rainfall recharge thresholds, including their spatial and temporal heterogeneity, is of fundamental importance to better understand recharge processes and improving estimation of recharge rates. Caves provide a unique observatory into the percolation of water from the surface to the water table at the timescale of individual rainfall recharge events. Here, we monitor nine infiltration sites over six years at a montane cave site in south eastern Australia. Six of the drip hydrology time series have up to ~100 hydrograph responses to rainfall over the monitoring period, three sites do not respond to rainfall events. We use two approaches to quantify rainfall recharge thresholds. At an annual timescale, for all nine drip sites, the total annual percolation water volume was determined for each year of data. Daily rainfall recharge thresholds were then determined by maximising the correlation of annual percolation water volume and total precipitation above a variable daily threshold value. The annual recharge amount methodology produced rainfall recharge thresholds for seven sites, where high and significant correlations (rank correlations > 0.75) occur for daily precipitation thresholds between 6 mm and 38 mm/day. No rainfall recharge thresholds could be obtained from one site which had a low and constant annual drip amount, and from one site which exhibited ‘underflow’ behaviour. At an event timescale, for the six sites which had a hydrograph response to rainfall, the 7-day antecedent rainfall amounts were determined. Minimum 7-day precipitation amounts prior to a hydrograph response for specific drip sites were in the range 13–28 mm and 75% of all recharge events had a 7-day antecedent precipitation between 20.7 and 38.1 mm. Combining all drip water monitoring sites and analysing the data by month identifies a seasonal variability in the minimum 7-day antecedent precipitation necessary to generate potential recharge, from 15 to 25 mm in winter to >50 mm in February and March. We apply a simple water budget model, driven by P and ET and optimised to the observed potential recharge events, to infer a ‘whole cave’ soil and epikarst storage capacity. This storage capacity is between ~50 mm (using potential evapotranspiration, 92% of events simulated successfully) to ~60 mm (using actual evapotranspiration, 79% of events simulated successfully). Modelling of individual drip sites identifies spatial heterogeneity in soil and epikarst storage capacities. Our approach using multiple methodologies allows the comparison between both daily and weekly rainfall recharge thresholds and modelled soil and epikarst storage for the first time.

Time lapse electric resistivity tomography to portray infiltration and hydrologic flow paths from surface to cave

This study provides the example of Golgotha Cave, in the south west of Western Australia, where previous long-term hydrological, geochemical and climate monitoring resulted in a theoretical hydrological model of the karst critical zone. We test this model by presenting the results of a 1.5 year monitoring program above the cave using time-lapse electric resistivity tomography (TL-ERT). Between May 2016 and January 2018 several lines up to 200 m long were monitored using a 100 multi-electrode Lipmann 4point light instrument. Various Wenner and Dipole-Dipole arrays provided information of electric resistivity changes at certain points of the year. Laboratory studies measuring volumetric water content versus electric resistivity of various rocks and soils resulted in three different ERT-hydrogeological zones. Besides the annual infiltration patterns, TL-ERT also identified previously unknown, permanent water storages as well as several areas acting as fast hydrological flow pathways most likely caused by pipe structures in the aeolianite. Specific aspects of the TL-ERT study could be correlated with the long term drip water monitoring and thus confirmed and extended the hydrogeological model of the area. A resulting conceptual model of the flow pathways derived from ERT surveys and cave water monitoring results is presented to visualise water flux from the surface to the ceiling of Golgotha Cave. This includes annual recharge to 40 m depth in less than one hydrological year, preferential flow paths to a persistent store of water, and a time lag between soil moisture saturation and seasonal connection of karst stores that indicates a narrower window for seasonal recharge than indicated by soil moisture data.

Drip water δ18O variability in the northeastern Yucatán Peninsula, Mexico: Implications for tropical cyclone detection and rainfall reconstruction from speleothems

This study examines the oxygen isotopic composition (δ18O values) of drip water, rainfall, and groundwater in the Río Secreto cave system, located in the Yucatán Peninsula, Mexico. The main motivation of this study was to determine the implications of drip water hydrology for the reconstruction of rainfall, droughts and tropical cyclone activity from stalagmite δ18O records. Monitoring of environmental and isotopic conditions was conducted for two years, from June 2017 to April 2019. This study provides the first instrumental evidence of an “amount effect” on interannual timescales in the Yucatán Peninsula. Observed bi-weekly to interannual variability in drip water δ18O values can be explained for individual drips by different integrations of rainfall amount in the time domain. Drip sites in two chambers (Stations A and B) integrate 4–15 months of rainfall accumulation. In a third chamber (Station LF) one drip site reflects the annual rainfall isotopic cycle with a positive offset and another, the largest rainfall events. During epikarst infiltration, the integration of rainfall amount by drip water source reservoirs determines the degree to which they “dilute” a tropical cyclone (TC) isotopic signature. TCs can be detected particularly when: (1) the water volume of the reservoir is low, such as during a persistent meteorological drought, and; (2) TCs have a sufficiently distinct isotopic signal relative to that of the reservoir prior to the event. TC isotopic signals can be masked or attenuated when drip water samples integrate more than a week and if significant rainfall events proceed the TC. In Río Secreto cave, reconstructing precipitation amount and detecting the TC isotopic signatures from stalagmite δ18O records is possible. Our analysis shows that stalagmite δ18O records are more likely to underestimate the magnitude of annual-scale droughts following normal hydroclimate conditions and more likely to record TCs during multiyear droughts than during normal or wet periods. Drip water monitoring results suggest that available stalagmite δ18O records from the Maya lowlands might be underestimating the intensity of paleo-drought events, such as the Terminal Classic droughts associated with the disintegration of the Maya civilization. This study complements the results from Lases-Hernandez et al. (2019) comparing two different sampling protocols of drip water collection. This study shows that a discrete sampling protocol is expected to approximate the amount-weighted isotopic composition of a drip, as long as it is conducted at a temporal resolution higher than the rainfall integration time by the drip reservoir. We highlight the importance of conducting multiyear monitoring of drip water and rainfall in order to interpret stalagmite δ18O as a paleoclimate proxy.

Rainfall recharge thresholds in a subtropical climate determined using a regional cave drip water monitoring network

Quantifying the combination of climatic and hydrological conditions required to generate groundwater recharge is challenging, yet of fundamental importance for groundwater resource management. Here we demonstrate a new unsaturated zone physical method of determining rainfall-recharge thresholds in karst using a regional cave drip water monitoring network. For limestones of the Upper and Lower Macleay Valley, eastern Australia, set in a subtropical climate, we observe thirty-one cave drip water recharge events over a five-year monitoring period. Comparison to antecedent precipitation demonstrates a median observed recharge threshold of 76 mm/week precipitation (Lower Macleay) and 79 mm/week precipitation (Upper Macleay), with lower precipitation thresholds (down to 30 mm/week) possible. We use a simple water budget model to quantify soil and epikarst water storage volumes and to test hypotheses of the hydrological controls. Modelled soil and epikarst water storage capacities of about 65 mm (Lower Macleay) and 80 mm (Upper Macleay) confirm a correspondence between observed weekly precipitation thresholds and soil and epikarst capacities. However, discrepancies between observed and simulated recharge events helps elucidate the likely recharge processes including focussed recharge bypassing the soil and epikarst store, overflow and drainage between multiple karst stores, and tree water use from depth. Our observed recharge thresholds and modelled soil and epikarst storage capacities are comparable to recharge thresholds estimated across a range of water-limited environments globally. The method is readily applicable to any karst region where drip loggers can be installed in a cave system in close proximity to surface climate data.

Extended Cave Drip Water Time Series Captures the 2015–2016 El Niño in Northern Borneo

AbstractTime series of cave drip water oxygen isotopes (δ18O) provide site‐specific assessments of the contributions of climate and karst processes to stalagmite δ18O records employed for hydroclimate reconstructions. We present ~12‐year‐long time series of biweekly cave drip water δ18O variations from three sites as well as a daily resolved local rainfall δ18O record from Gunung Mulu National Park in northern Borneo. Drip water δ18O variations closely match rainfall δ18O variations averaged over the preceding 3–18 months. We observe coherent interannual drip water δ18O variability of ~3‰ to 5‰ related to the El Niño–Southern Oscillation (ENSO), with sustained positive rainfall and drip water δ18O anomalies observed during the 2015/2016 El Niño. Evidence of nonlinear behavior at one of three drip water monitoring sites implies a time‐varying contribution from a longer‐term reservoir. Our results suggest that well‐replicated, high‐resolution stalagmite δ18O reconstructions from Mulu could characterize past ENSO‐related variability in regional hydroclimate.

Lignin oxidation products as a potential proxy for vegetation and environmental changes in speleothems and cave drip water – a first record from the Herbstlabyrinth, central Germany

Abstract. Here, we present the first quantitative speleothem record of lignin oxidation products (LOPs), which has been determined in a Holocene stalagmite from the Herbstlabyrinth Cave in central Germany. In addition, we present LOP results from 16 months of drip water monitoring. Lignin is only produced by vascular plants and therefore has the potential to be an unambiguous vegetation proxy and to complement other vegetation and climate proxies in speleothems. We compare our results with stable isotope and trace element data from the same sample. In the stalagmite, LOP concentrations show a similar behavior to P, Ba and U concentrations, which have previously been interpreted as vegetation proxies. The LOP S∕V and C∕V ratios, which are usually used to differentiate between angiosperm and gymnosperm and woody and non-woody vegetation, show complex patterns suggesting additional influencing factors, such as transport and microbiological effects. The drip water from a fast drip site shows a seasonal pattern of LOPs with low LOP concentrations in winter and higher LOP concentrations in summer. These results indicate the potential of LOPs as a new proxy for vegetational and environmental changes in speleothems but also demonstrate the complexity and the current limitations of our understanding of the transport of lignin from the soil into the cave and the speleothems.

Cave drip water solutes in south-eastern Australia: Constraining sources, sinks and processes

Constraining sources and site-specific processes of trace elements in speleothem geochemical records is key to an informed interpretation. This paper examines a 10-year data set of drip water solutes from Harrie Wood Cave, south-eastern Australia, and identifies the processes that control their response to El Niño-Southern Oscillation events which varies the site water balance. The contributions of aerosol and bedrock end-members are quantified via hydrochemical mass balance modelling. The parent bedrock is the main source for the drip water solutes: Mg, Sr, K and trace elements (Ba, Al, V, Cr, Mn, Ni, Co, Cu, Pb and U), while atmospheric aerosol inputs also contribute significantly to drip water trace elements and Na, K and Zn. A laboratory investigation evaluating water-soluble fractions of metals in soil samples and soil enrichment factors provided a basis for understanding metal retainment and release to solution and transport from the soil zone. These results identified the role of the soil as a sink for: trace metals, Na and K, and a secondary source for Zn. Further, soil processes including: cation exchange, K-fixation, metal adsorption to colloids and the release of Zn associated with organic matter degradation further modify the chemical composition of the resultant drip waters. This research is significant for the south-eastern Australian region, as well as other sites in a karst setting with clay-rich soil. In particular these results reveal that the response of drip water chemistry to hydroclimatic forcing is non-linear, with the greatest response observed when the long-term gradient in the cumulative water balance reverses. This longer-term drip water monitoring dataset is significant because it provides the pivotal framework required to reliably identify suitable trace element proxies for interpretation in geochemical speleothem records on multi-decadal timescales.

Links between host rock, water, and speleothems of Xueyu Cave in Southwestern China: lithology, hydrochemistry, and carbonate geochemistry

A systemic study was conducted to understand the links between the host rock, water, and speleothems of Xueyu Cave in Southwestern China. An outcrop section of the host rock of Xueyu Cave was surveyed, the stratum that the host rock belongs to was subdivided into 11 lithofacies, and the Xueyu Cave is developed in the grainstone facies segment. Lithological characteristics and chemical components of the host rock indicate that its predominant mineral component is low magnesium calcite (LMC). To understand the hydrochemical characteristics of percolating water in Xueyu Cave, three drip water monitoring sites were taken to collect water samples for a whole monsoon year. The temperature (T), pH, electronic conductivity (EC), drip rate, Ca2+, Mg2+, Na+, K+, Ba2+, Sr2+, HCO3 −, SO4 2−, NO3 −, Cl−, partial pressure of CO2 (pCO2), saturation indexes, and precipitation rate of carbonate minerals were investigated monthly. The data reveals that the pH, EC, drip rate, Ca2+, Mg2+, HCO3 −, pCO2, saturation indexes, and precipitation rate of carbonate minerals exhibit seasonal variation; these are due to the seasonal change of precipitation and soil CO2 concentration that are driven by monsoon climate. The petrological and geochemical characteristics of speleothems were also investigated, and results indicate that the main chemical and mineral components of speleothems are quite similar to the host rock; this is because the main material source of drip water and speleothems are governed by the host rock. This study reveals the main mineralogical and geochemical characteristics of host rock and speleothems, further discussing the seasonal variation of hydrochemical indexes of drip water. All these works will be helpful to more thoroughly understand the operation regularity of karst dynamic system (KDS) of the Xueyu Cave.