Mass Balance Techniques Research Articles

15N mass balance technique for measuring ammonia losses from soil surface‐applied slurries containing various additives

AbstractBackgroundAnthropogenic ammonia emissions, primarily derived from agriculture, lead to air pollution, soil acidification, and surface water eutrophication, all of which adversely affect human health and ecosystems. Slurry treatment technologies in the form of additives represent an underutilized means of reducing gaseous emissions. Information regarding the potential of additives to reduce ammonia in soil surface‐applied slurries is scarce.AimThis study aims to develop a 15N mass balance technique to quantitatively measure ammonia losses from different slurries containing multiple additives that are applied to outdoor soil‐filled containers.MethodsThe experiments were performed under free‐air conditions. Isotopically labeled slurries from biogas, cattle, and pigs containing 18 additives were surface‐applied to soil‐filled containers and exposed for 72 or 48 h. The additives included inorganic and organic adsorbents, five amounts of sulfuric acids, molasses ± effective microorganisms, and water dilution. After termination of the ammonia loss period, a suite of soil preparation steps for the quantitative recovery of the labeled ammonium remaining in the soil was developed, and subsequently the loss of NH4‐N was determined.ResultsIn the control treatments, ammonia losses from biogas, cattle slurries, and pig slurries averaged 54.4%, 33.9%, and 11.0%, respectively. The adsorbents did not decrease or only slightly decreased ammonia emissions. Ammonia abatement by sulfuric acid was nearly complete at pH values of 5.9 and 5.8 for the biogas and pig slurry, respectively, and about 80% at pH 5.2 for the cattle slurry. In comparison, more moderately decreased pH values with sulfuric acid showed a similar reduction as molasses and a 1:1 dilution for the three slurries. Adding microorganisms to the molasses did not further decrease ammonia losses.ConclusionThe newly developed 15N mass balance technique, which allows a precise estimate of ammonia losses, can serve as a reference method to assess ammonia losses from field‐applied slurries containing various additives and as a standard comparison technique for other ammonia measurement techniques.

Optimizing an airborne mass-balance methodology for accurate emission rate quantification of industrial facilities: A case study of industrial facilities in South Korea

Accurate estimation of emissions from industrial point sources is crucial in understanding the effectiveness of reduction efforts and establishing reliable emission inventories. In this study, we employ an airborne Chemical Ionization Mass Spectrometry (CIMS) instrument to quantify sulfur dioxide (SO2) emissions from prominent industrial facilities in South Korea, including power plants, a steel mill, and a petrochemical facility. Our analysis utilizes the box mass balance technique to derive SO2 emissions and associated uncertainty. We evaluate the interpolation methods between 2D kriging and 3D radial basis function. The results demonstrate that the total uncertainty of the box mass balance technique ranges from 5 % to 28 %, with an average of 20 %. Mixing ratio ground extrapolation from the lowest altitude of the airborne sampling to the ground emerges as the dominant source of uncertainty, followed by the determination of the boundary layer height. Adequate sampling at multiple altitudes is found to be essential in reducing the overall uncertainty by capturing the full extent of the plume. Furthermore, we assess the uncertainty of the single-height transect mass balance method commonly employed in previous studies. Our findings reveal an average precision of 47 % for this method, with the potential for overestimating emissions by up to 206 %. Samplings at fewer altitudes or with larger altitude gaps increase the risk of under-sampling and elevate method uncertainties. Therefore, this study provides a quantitative basis to evaluate previously airborne observational emission constraints.

A Novel High‐Resolution In Situ Tool for Studying Carbon Biogeochemical Processes in Aquatic Systems: The Lake Aiguebelette Case Study

AbstractLakes and reservoirs are a significant source of atmospheric methane (CH4), with emissions comparable to the largest global CH4 emitters. Understanding the processes leading to such significant emissions from aquatic systems is therefore of primary importance for producing accurate projections of emissions in a changing climate. In this work, we present the first deployment of a novel membrane inlet laser spectrometer (MILS) for fast simultaneous detection of dissolved CH4, ethane (C2H6) and the stable carbon isotope of methane (δ13CH4). During a 1‐day field campaign, we performed 2D mapping of surface water of Lake Aiguebelette (France). Average dissolved CH4 concentrations and δ13CH4 were 391.9 ± 156.3 nmol L−1 and −67.3 ± 3.4‰ in the littoral area and 169.8 ± 26.6 nmol L−1 and −61.5 ± 3.6‰ in the pelagic area. The dissolved CH4 concentration in the pelagic zone was 50 times larger than the concentration expected at equilibrium with the atmosphere, confirming an oversaturation of dissolved CH4 in surface waters over shallow and deep areas. The results suggest the presence of CH4 sources less enriched in 13C in the littoral zone (presumably the littoral sediments). The CH4 pool became more enriched in 13C with distance from shore, suggesting that oxidation prevailed over epilimnetic CH4 production and it was further confirmed by an isotopic mass balance technique with the high‐resolution data. This new in situ fast response sensor allows one to obtain unique high‐resolution and high‐spatial coverage data sets within a limited amount of survey time. This tool will be useful in the future for studying processes governing CH4 dynamics in aquatic systems.

Investigation of the Effect of Residual Oil and Wettability on Sulfonated Polymer Retention in Carbonate under High-Salinity Conditions

Summary Polymer flooding in carbonate reservoirs is greatly affected by polymer retention, which is mainly due to adsorption by polymer-rock surface interactions. Consequently, this leads to a delay in polymer front propagation and related oil recovery response. This work investigates the effects of residual oil (Sor) and wettability on sulfonated-based (ATBS) polymer retention under the conditions of high salinity and moderate temperature. Polymer single- and two-phase dynamic adsorption tests as well as bulk and in-situ rheological experiments were conducted on outcrop carbonate cores in the presence of a high-salinity brine of 243,000 ppm at a temperature of 50°C. A total of four corefloods were conducted on Indiana limestone core samples with similar petrophysical properties. Overall, polymer adsorption was found to be low and within the acceptable range for application in carbonate reservoirs in the absence and presence of Sor. Furthermore, the polymer adsorption and in-situ rheology tests highlighted the significance of oil presence in the core samples, where retention was found to be around 40–50 µg/g-rock and 25–30 µg/g-rock in the absence and at Sor, respectively. An additional 50% reduction in retention was observed on the aged core sample that is more oil-wet. Polymer retention/adsorption was measured by double slug and mass balance techniques, and the results from both methods were in agreement with less than 7% difference. Inaccessible pore volume (IPV) was also calculated based on the double slug method and was found to be in the range of 23% to 28%, which was qualitatively supported by in-situ saturation monitoring obtained from an X-ray computed tomography (CT) scanner. The ATBS-based polymer showed excellent results for applications in carbonate without considerable polymer loss or plugging. This paper provides valuable insights into the impacts of residual oil and wettability on polymer adsorption, supported by CT in-situ saturation monitoring, which is necessary to avoid unrepresentative and inflated polymer retentions in oil reservoirs.

Assessing Carbon Emissions from Biomass Burning in Croplands in Burkina Faso, West Africa

Agricultural biomass burning plays a critical role in carbon emissions, with implications for climate change. This study aims to assess carbon (C) emissions and establish C, CO, CO2 and CH4 emission factors (EFs) by simultaneously testing the effects of climatic conditions and cropland category on gas emissions. In Burkina Faso, 96 experimental fires were conducted in accordance with farmers’ operations during the land-clearing season in two climatic zones (Sudanian and Sudano-Sahelian) and across two cropland categories (Cropland Remaining Cropland (CC) and Land Converted to Cropland (LC)). The carbon mass balance technique was applied to estimate emissions. Climate zone and cropland category significantly influenced carbon emissions and emission factors (p < 0.05). The Sudanian zone recorded the highest carbon emissions (0.24 ± 0.01 t C ha−1). For cropland category, LC recorded the highest carbon emissions with an average value of 0.27 ± 0.01 t C ha−1. CO2 EFs ranged from 1661.44 ± 3.63 g kg−1 in the Sudanian zone to 1716.51 ± 3.24 g kg−1 in the Sudano-Sahelian zone. EFs showed a dependence on the cropland category, with the highest EFs in CC. Smart agricultural practices limiting cropland expansion and biomass burning need to be promoted. This study provides vital information useful for supporting decision making as part of Nationally Determined Contributions.

Drivers of hydrologic budgets in small terminal lakes in the Alberta prairies

The prairies and boreal plain within the North Saskatchewan Watershed (NSW) of Central Alberta have numerous shallow ponds and lakes that sustain unique aquatic ecosystems and are critical habitat for migratory waterfowl in North America. However, over the past 20 years water levels have declined and the reasons are unresolved. Here we used a combination of inorganic geochemical analyses and stable water isotopes to constrain the hydrologic budgets of six lakes in the NSW. Our results show that the bedrock groundwater major element geochemistry is controlled by chemical weathering reactions along the flow paths and is dominated by lower δ18O and δ2H values (i.e. isotopically depleted), while the lake water generally shows unchanging evaporatively enriched stable isotope values and cation concentrations. An isotopic mass balance (IMB) technique combined with solution geochemical modelling using activity – activity plots reveals that deep groundwater input is negligible, while the lakes appear to lose a greater fraction of water inflows to evaporation (60%) than shallow groundwater and surface outflow (40%). The relative importance of shallow groundwater requires further study, as shallow groundwater sampling locations are scarce and surface outflow is negligible. The IMB technique also indicated that these prairie lakes have short water residence times, ranging from 1.8 to 10.4 yrs. Our results suggest that declining lake levels are likely the result of a changing relationship between precipitation and evaporation from the climatic norm.

Technical note: Isolating methane emissions from animal feeding operations in an interfering location

Abstract. Agricultural emissions, including those from concentrated animal feeding operations (CAFOs) for beef and dairy cattle, make up a large portion of the United States' total greenhouse gas (GHG) emissions. However, many CAFOs reside in areas where methane (CH4) from oil and natural gas (ONG) complicates the quantification of CAFO emissions. Traditional approaches to quantify emissions in such regions often relied on inventory subtraction of other known sources. We compare the results of two approaches to attribute the CAFO CH4 emission rate from the total CH4 emission rate derived from an aircraft mass balance technique. These methods make use of the mixing ratio data of CH4, ethane (C2H6), and ammonia (NH3) that were collected simultaneously in-flight downwind of CAFOs in northeastern Colorado. The first approach, the subtraction method (SM), is similar to inventory subtraction, except the amount to be removed is derived from the observed C2H6 to CH4 ratio rather than an inventory estimate. The results from this approach showed high uncertainty, primarily due to how error propagates through subtraction. Alternatively, multivariate regression (MVR) can be used to estimate CAFO CH4 emissions using the NH3 emission rate and an NH3 to CH4 ratio. These results showed significantly less uncertainty. We identified criteria to determine the best attribution method; these criteria can support attribution in other regions. The final emission estimates for the CAFOs presented here were 13 ± 3 g of CH4 per head per hour and 13 ± 2 g of NH3 per head per hour. These estimates are higher than the inventory of the US Environmental Protection Agency (EPA) and previous studies highlighting the need for more measurements of CH4 and NH3 emission rates.

Assessing Feasibility of Water Resource Protection Practice at Catchment Level: A Case of the Blesbokspruit River Catchment, South Africa

The operationalization of water resource protection initiatives for surface water resource quality and equitable water quality allocation is critical for sustainable socio-economic development. This paper assessed Blesbokspruit River Catchment’s water quality status, using the South African Water Quality standards and Water Quality Index (WQI). Protection levels for quality, and waste discharge for point sources were set and evaluated using the total maximum daily loads (TMDLs) and chemical mass balance (CMB) techniques, respectively. The study found that the water quality results for the analysed physico-chemical parameters (Na+, Ca2+, Mg2+, Cl−, F−, pH, EC, SO42−) of the data collected from 2015 to 2022 were within the limits of the water quality standards, except for NO3− and PO42−. The water quality from the study area was categorized as acceptable for drinking purposes with the WQI of 54.80. The application of the TMDL approach resulted in the 77.96 mS/m for electrical conductivity (EC), 9.92 mg/L for phosphate (PO42−), and 15.16 mg/L for nitrate NO3− being set as the protection levels for the catchment. The CMB was found to be a useful tool for the evaluation of point source discharges into water resources. The study recommends the application of TMDL and CMB techniques in water resource protection practice.

Assessing the bias and uncertainties in the aircraft mass balance technique for the determination of carbon dioxide emission rates

Urban areas are the major sources of greenhouse gas emissions but also leaders in emission reduction efforts. Appropriate techniques to quantify emissions and any potential reductions over time are necessary to effectively inform these mitigation efforts. The aircraft mass balance experiment (MBE) is an established technique used for such a purpose. In this work, we use a series of 55 MBEs downwind of power plants to assess the technique’s bias and precision. In addition, we investigate what factors drive the absolute error, determined as the absolute difference between observed and reported emission rates, in individual experiments using multilinear regressions. Power plants are required to monitor their carbon dioxide emissions with an hourly resolution, and these publicly available reported emissions can be directly compared to the mass balance estimates as a pseudo-known release. To quantify the bias we calculated the mean error, which was 10 ± 240 Mg·h−1 (1σ), regressed mass balance emission rates against reported emission rates to yield a slope of 0.967 ± 0.062, and compared the sum across all mass balance emission rates, 31,000 ± 1,000 Mg·h−1, to the sum across all reported emissions, 30,660 ± 740 Mg·h−1. All three of these approaches suggest no systematic bias. Then to quantify the precision for individual determinations we calculated the slope of a regression between the standard deviation across repeated MBEs and the corresponding average emission rate, which is 30.7% ± 6.7%. The main drivers of the absolute error were sparse sampling of the plume, poor horizontal and vertical mixing of the plume, and smaller signal-to-noise ratios. Quantifying the capabilities of this technique provides context for previous analyses and allows stakeholders and researchers to make informed decisions when choosing quantification methods. Identifying the factors that drive the absolute error also allows us to adjust flight design to minimize it and potentially improve uncertainty estimates.

Methods for the SI-traceable value assignment of the purity of organic compounds (IUPAC Technical Report)

Abstract The “purity” of an organic compound typically refers, in practice, to an assignment of the mass fraction content of the primary organic component present in the material. The “purity” value of an organic primary calibrator material is the ultimate source of metrological traceability of any quantitative measurement of the content of that compound in a given matrix. The primary calibrator may consist of a Certified Reference Material (CRM) whose purity has been assigned by the CRM producer or a laboratory may choose to value-assign a material to the extent necessary for their intended application by using appropriately valid methods. This report provides an overview of the approach, performance and applicability of the principal methods used to determine organic purity including mass balance, quantitative NMR, thermal methods and direct-assay techniques. A statistical section reviews best practice for combination of data, value assignment as the upper limit values corresponding to 100 % purity are approached and how to report and propagate the standard uncertainty associated with the assigned values.

Estimation of Groundwater Recharge in Makutupora Basin Located in a Semi-arid Region in Central Tanzania using Soil Moisture Balance (SMB) and Chloride Mass Balance (CMB) Techniques

ABSTRACT The objective of this study is to assess the groundwater recharge in the Makutupora basin, Central Tanzania using the soil moisture balance (SMB) and chloride mass balance (CMB) methods. The SMB method considered the estimation of both evapotranspiration in CROPWAT 8.0 software and surface runoff by the curve number (CN) method. The CN of the basin was determined based on hydrological soil group (HSG), antecedent moisture condition (AMC) and land use type using Arc GIS 10.2. Distributed land-use maps, slope and soil texture maps, daily climatological data (e.g., precipitation, temperature, wind speed, etc.), chloride concentration in rainfall and groundwater are the basic data used in this study. Input data were prepared in the form of digital maps using GIS and remote sensing tools and finally excel models were created to compute recharge using a set of given conditions. The results indicate that the average chloride concentration measured in the groundwater samples of the study area is 282.88 mg/L; much higher than the average concentration in rainwater samples which is 3.47mg/L while the average potential evapotranspiration is 2280 mm/year with an average rainfall of 680mm/year. Runoff computed using the CN method is ranging from 7.6-246mm/year or 0.68-22%. The final recharge from SMB is ranging from 0.025-117.86mm/year or 0.006-10% of annual rainfall equivalent to an average annual volume of 41.58Mm3 while recharge from CMB ranges from 2.57-44.6mm/year or 0.38-6.56% of annual rainfall equivalent to an average annual volume of 23.85Mm3. The results show that CMB underestimated the recharge compared to the SMB method. The reason for this is thought to be caused by the high chloride concentration analysed in groundwater compared to rainfall samples which may be caused by increased chloride concentration as water passes through the rock formations (unsaturated zone) to the saturated zone. Further lack of atmospheric chloride deposition data may be another contributing factor.

Groundwater recharge using the chloride mass balance method in the Kanye area, in southeast Botswana

This paper reports on groundwater recharge evaluation conducted in the area around Kanye village in southeast Botswana, using the chloride mass balance (CMB) technique. Thirty-seven (37) years of rainfall data were examined to define the seasonal distribution and the average rainfall in the study area. The rainfall pattern through the twelve months of each year was further analysed using rainfall coefficient techniques. Thirteen (13) groundwater samples from different well-fields were collected during the rainy and dry seasons, and analysed for chloride and bromide concentrations. Additionally, chloride concentrations of the rainfall during the wet and dry seasons were obtained from a previous groundwater recharge evaluation study conducted in the same region. Two rainy seasons and one dry season are recognised in the Kanye area. The average annual rainfall in Kanye is 466 mm. 57% of this occurs in the first rainy season (January to April); 37% occurs in the second rainy season (October to December), and 6% occurs in the dry season (May to September). The CMB method reveals that the groundwater recharge ranges from 4.8 to 26.6 mm/yr. The recharge in the first rainy season is estimated to be 5.70%, whereas the recharge in the second rainy season and the dry season are 4.09 and 1.03%, respectively, of the average rainfall. The results show that large amounts of water are lost from the area in different forms after the rains. Harvesting the water lost as surface runoff could enhance the groundwater recharge of the Kanye area.

Quantifying Dynamic Water Storage in Unsaturated Bedrock with Borehole Nuclear Magnetic Resonance

AbstractQuantifying the volume of water that is stored in the subsurface is critical to studies of water availability to ecosystems, slope stability, and water‐rock interactions. In a variety of settings, water is stored in fractured and weathered bedrock as rock moisture. However, few techniques are available to measure rock moisture in unsaturated rock, making direct estimates of water storage dynamics difficult to obtain. Here, we use borehole nuclear magnetic resonance (NMR) at two sites in seasonally dry California to quantify dynamic rock moisture storage. We show strong agreement between NMR estimates of dynamic storage and estimates derived from neutron logging and mass balance techniques. The depths of dynamic storage are up to 9 m and likely reflect the depth extent of root water uptake. To our knowledge, these data are the first to quantify the volume and depths of dynamic water storage in the bedrock vadose zone via borehole NMR.

Development and evaluation of a processing sector model for butter manufacture using a mass balance technique at two dairy processing sites

The butter manufacturing process at two different commercial dairy processing sites in Ireland was evaluated using a mass balance approach to develop, evaluate and validate a processing sector model of the flow of milk fat from intake to final product. The mass balance was represented as a function of fat intake = fat in products + fat losses + recycled fat. Representative samples of all products, namely whole milk, cream, skim milk, butter, buttermilk and cleaning‐in‐place streams (cream silo flush, butter churn residue and sludge), were collected from two different sites. Milk fat levels and product quantities were measured to obtain the fat outputs. Total fat losses at the end of butter production ranged between 1.90% and 2.25% of the total fat input for both sites. Three different scenarios were examined to evaluate the model: S1 (Animal Breed) high genetic merit (Elite) and national average (NA) Holstein Friesian (HF) cows were evaluated, for their effect on the net value of milk; S2 (Product Portfolio) a mixed product portfolio of cheese, butter and skim milk powder (SMP) was compared to a product portfolio comprised of butter alone; and S3 (Process Efficiency) the impact of varying process losses on net values of milk and the quantities of products produced was simulated. The value per 1000 L of milk for S1 was €410.69 and €393.20 for Elite and NA cow’s milk, respectively. For S2, the butter‐only product portfolio returned €355.10, whereas the mixed‐products portfolio returned €369.60. Lastly, S3 corresponding returns for 1%, 2.2% and 5% losses was €365.90, €361.47 and €351.12, respectively.

Evaluating Cropland N2O Emissions and Fertilizer Plant Greenhouse Gas Emissions With Airborne Observations

AbstractAgricultural activity is a significant source of greenhouse gas emissions. The fertilizer production process emits N2O, CO2, and CH4, and fertilized croplands emit N2O. We present continuous airborne observations of these trace gases in the Lower Mississippi River Basin to quantify emissions from both fertilizer plants and croplands during the early growing season. Observed hourly emission rates from two fertilizer plants are compared with reported inventory values, showing agreement for N2O and CO2 emissions but large underestimation in reported CH4 emissions by up to a factor of 100. These CH4 emissions are consistent with loss rates of 0.6–1.2%. We quantify regional emission fluxes (100 km) of N2O using the airborne mass balance technique, a first application for N2O, and explore linkages to controlling processes. Finally, we demonstrate the ability to use airborne measurements to distinguish N2O emission differences between neighboring fields, determining we can distinguish different emission behaviors of regions on the order of 2.5 km2 with emissions differences of approximately 0.026 μmol m−2 s−1. This suggests airborne approaches such as outlined here could be used to evaluate the impact of different agricultural practices at critical field‐size spatial scales.

Assessment of Regional Methane Emission Inventories through Airborne Quantification in the San Francisco Bay Area.

This study derives methane emission rates from 92 airborne observations collected over 23 facilities including 5 refineries, 10 landfills, 4 wastewater treatment plants (POTWs), 2 composting operations, and 2 dairies in the San Francisco Bay Area. Emission rates are measured using an airborne mass-balance technique from a low-flying aircraft. Annual measurement-based sectorwide methane emissions are 19,000 ± 2300 Mg for refineries, 136,700 ± 25,900 Mg for landfills, 11,900 ± 1,500 Mg for POTWs, and 11,100 ± 3,400 Mg for composting. The average of measured emissions for each refinery ranges from 4 to 23 times larger than the corresponding emissions reported to regulatory agencies, while measurement-derived landfill and POTW estimates are approximately twice the current inventory estimates. Significant methane emissions at composting facilities indicate that a California mandate to divert organics from landfills to composting may not be an effective measure for mitigating methane emissions unless best management practices are instituted at composting facilities. Complementary evidence from airborne remote sensing imagery indicates atmospheric venting from refinery hydrogen plants, landfill working surfaces, composting stockpiles, etc., to be among the specific source types responsible for the observed discrepancies. This work highlights the value of multiple measurement approaches to accurately estimate facility-scale methane emissions and perform source attribution at subfacility scales to guide and verify effective mitigation policy and action.

Nitrogen transformation in engineered aquaponics with water celery (Oenanthe javanica) and koi carp (Cyprinus carpio): Effects of plant to fish biomass ratio

Nitrogen plays critically important role in dual biomasses aquaponic production. This study examined the fate of nitrogen in an aquaponics at different plant-fish (P/F) biomass ratios. A floating raft hydroponic bed with water celery (Oenanthe javanica) as plant species coupled with an aquaculture tank reared with Koi carp (Cyprinus carpio) was evaluated using nitrogen mass balance techniques at P/F biomass ratio of 0.06–0.95 and feed conversion ratios (FCR) maintained at 1.73–1.85 at a constant hydraulic retention time of 24 h. The FCRs in the study were significantly lower than FCR (1.96) in the control (aquaculture only). Nitrogen use efficiency (NUE) in the aquaponics increased from 29.3 ± 3.5 to 39.1 ± 4.2% as the P/F biomass ratio increased from 0.06 to 0.95, respectively. Furthermore, the increase in P/F biomass ratio to 0.95 exhibited the highest nitrogen uptake rate by plants (~56 mg/day in the hydroponic bed) which resulted in a levelling-off of the nitrate concentration at 62.5 ± 4.2 mg/L in the aquaculture tank. The increase in P/F biomass ratio also lowered the nitrous oxide (N2O) emissions by 17% from the aquaponics compared to control. Overall, this study presents evidence that an aquaponics with higher P/F biomass ratio emits less N2O and has higher nutrient recovery with perpetual fish production in a zero water exchange-aquaponic system.

Air–sea gas exchange at wind speeds up to 85 m s−1

Abstract. Gas transfer velocities were measured in two high-speed wind-wave tanks (Kyoto University and the SUSTAIN facility, RSMAS, University of Miami) using fresh water, simulated seawater and seawater for wind speeds between 7 and 85 m s−1. Using a mass balance technique, transfer velocities of a total of 12 trace gases were measured, with dimensionless solubilities ranging from 0.005 to 150 and Schmidt numbers between 149 and 1360. This choice of tracers enabled the separation of gas transfer across the free interface from gas transfer at closed bubble surfaces. The major effect found was a very steep increase of the gas transfer across the free water surface at wind speeds beyond 33 m s−1. The increase is the same for fresh water, simulated seawater and seawater. Bubble-induced gas transfer played no significant role for all tracers in fresh water and for tracers with moderate solubility such as carbon dioxide and dimethyl sulfide (DMS) in seawater, while for low-solubility tracers bubble-induced gas transfer in seawater was found to be about 1.7 times larger than the transfer at the free water surface at the highest wind speed of 85 m s−1. There are indications that the low contributions of bubbles are due to the low wave age/fetch of the wind-wave tank experiments, but further studies on the wave age dependency of gas exchange are required to resolve this issue.

Comparison of isotopic mass balance and instrumental techniques as estimates of basin hydrology in seven connected lakes over 12 years

Mass-balance models using stable isotopes of hydrogen and oxygen provide useful estimates of the water balance of lakes, particularly in the absence of instrumental data. However, isotopic mass balances are rarely compared directly to measured water fluxes. Here we compared instrumental and isotope-based determinations of water fluxes in seven connected lakes over 12 years to quantify how agreement between the two approaches is affected by lake type and its position in the landscape. Overall, lake-specific ratios of evaporation to inflow (E/I) from instrumental measurements (median, x̃ = 0.06, median absolute deviation, MAD = 0.06) agreed well with isotopic estimates using headwater models (x̃ = 0.14, MAD = 0.08), with the exception of one lake with limited channelized inflow of surface waters (x̃instrumental = 0.51 vs. x̃headwater = 0.24). Isotope-instrument agreement improved (x̃ = 0.09 vs. x̃ = 0.03) when basin-specific (‘best-fit’) isotope models also considered local connectivity to upstream water bodies. Comparison among years revealed that mean isotopic E/I values were lowest in 2011 (mean, μ = 0.06, standard deviation, σ = 0.09) during a 1-in-140 year spring flood, and highest during a relatively arid year, 2003 (μ = 0.22, σ = 0.19), while interannual variability in E/I generally increased with distance downstream along the mainstem of the watershed. Similar patterns of agreement between methods were recorded for water-residence time. Isotope models also documented the expected low water yield from lake catchments (μ = 36.2 mm yr−1, σ = 62.3) suggesting that isotope models based on late-summer samples integrate annual inputs from various sources that are difficult to measure with conventional methods. Overall, the strong positive agreement between methods confirms that water isotopes can provide substantial insights into landscape patterns of lake hydrology, even in ungauged systems.

Characterisation of groundwater flow and recharge in crystalline basement rocks in the Talensi District, Northern Ghana

A numerical groundwater flow model, calibrated under equilibrium conditions to characterise the spatial variation of key hydraulic parameters and groundwater flow pattern in a crystalline aquifer in the Upper East Region of Ghana has been developed. The development of groundwater flow model was to provide the baseline information to assist in assessing the impacts of population growth and climate change scenarios on large scale development of groundwater resources for various uses. The results revealed an apparent dominant northeast–southwest flow pattern influenced mainly by the hydraulic conductivity field, with estimated values ranging between 0.001 and 58 m/day, and local flow systems which are controlled mainly by local variations in the topography in the area. The estimated average groundwater recharge from the calibrated model and chloride mass balance technique: 2.00% and 2.07% of the annual precipitation respectively has a direct positive correlation with elevation. The low recharge rate is evident in the high evaporation rates as revealed by stable isotope (δ2H and δ1⁸O) analysis of sampled water sources. The study also reveals the aquifer is being partly recharged by the White Volta River. This gives an indication that the area holds good groundwater fortunes for potential commercial groundwater development as predicted by scenario analysis which suggests that the aquifer can sustain increased abstractions by more than 100% of the current rate provided the current recharge rate is maintained. However, for the aquifer to sustain large scale abstractions beyond 100% of the current rate under conditions of reduced vertical recharge by more than 40% of the current rate, deliberate efforts would have to be made to enhance artificial vertical recharge to augment the natural recharge.