Detecting Freshwater Inputs via Groundwater Discharge to Marina Lagoon, Mediterranean Coast, Egypt

Much work in recent years has reported on the role of submarine groundwater discharge (SGD) on coastal biogeochemistry, but most of those studies have focused on temperate or tropical climates where year-round rainfall recharges surficial aquifers. The aim of this work, however, was to examine SGD behaviors in an arid setting??Marina Lagoon on the Egyptian Mediterranean coast. SGD was estimated via radon surveys and time-series measurements in lagoon waters during two campaigns (wet season in March 2009 and dry season in July 2010). Relatively higher values of radon were detected in March (maximum >30?dpm/L) compared to July (up to 16?dpm/L), which would indicate either enhanced input rates, or lower mixing/atmospheric losses during the wet season. Lower salinity waters within Marina Lagoon were characterized by higher radon and higher concentrations of dissolved inorganic nitrogen (DIN) and silica (DISi), characteristic of groundwater inputs. Based on lagoon and groundwater radon measurements and an advection-diffusion model, SGD average rates between 0.83 to 2.4?×?10⁸?L/day were estimated for both surveys. Since no seasonal pattern was detected, we suspect that either (1) SGD is derived from a regional aquifer not influenced by recharge from local rainfall or (2) local water use for irrigation and domestic purposes artificially recharges the surficial aquifer in the dry summer (tourist) months, which compensates for the lack of rainfall occurring at that time.     

Importance of submarine groundwater discharge as a source of nutrients for the Neuse River Estuary,North Carolina

Seepage rate and chemical composition of groundwater discharge entering the Neuse River Estuary (NRE) were quantified over an annual cycle from July 2005 through June 2006. Lee type seepage meters were deployed at eight locations within the NRE to quantify the amount of submerged groundwater discharge (SGD) entering the system. Sediment porewater nitrate (NO₃ ⁻), ammonium (NH₄ ⁺), and phosphate (PO₄ ⁻³) were also quantified at each of these locations to determine groundwater chemical composition. Seepage rates for the system ranged from 0.004 to 0.035 m³ m⁻² d⁻¹. Both the average and median value for the system-wide SGD were 0.01 m³ m⁻²d⁻¹. There were no significant differences between upstream and downstream seepage rates or between those at the north and south side of the estuary. Seepage rates varied greatly in time and space. Discharging groundwater was NO₃ ⁻ deplete but highly enriched in NH₄ ⁺. Porewater PO₄ ⁻³ levels varied but were usually present below Redfield values due to NH₄ ⁺ enrichment. SGD nutrient loading represented a small part of watershed nitrogen and phosphorus loading, 0.8% and 1.0%, respectively.     

Submarine groundwater discharge into Venice Lagoon, Italy

Venice Lagoon, Italy, rests on a series of aquifers that are 1,000 m thick. Measurements of submarine groundwater discharge (SGD) were made in Venice Lagoon using benthic chambers vented to a plastic collection bag. Two hundred measurements taken in a pristine northern lagoon site (Isola la Cura) revealed flow rates as high as 200 cm d⁻¹ with an average of 30 cm d⁻¹. Over 100 measurements taken adjacent to a bulkhead shoreline in the Porto Marghera industrial zone (Fusina) showed flow rates as high as 30 cm d⁻¹ and averaging 6 cm d⁻¹. These flow rates, if representative of even a fraction of the lagoon floor, are easily able to account for the 15% deficit previously calculated between precipitation and runoff for the entire Venice Lagoon drainage basin. Land elevation surrounding the Venice Lagoon is < 10 m within 20 km of the shoreline and is unable to support any substantial onshore water table. Submarine groundwater discharge most likely represents upward artesian discharge from deeper partially confined aquifers. Over 60 samples were collected in total from both sites for nutrient analysis. Ammonium concentration was found to be 2–8 fold higher in the device water than in the lagoon water at the northern site depending on season, and 10–30 times higher at the industrial zone site. These numbers suggest that SGD may be the primary pathway for nutrients and perhaps other contaminants to enter Venice Lagoon.     

Natural Radon and Radium Isotopes for Assessing Groundwater Discharge into Little Lagoon,AL: Implications for Harmful Algal Blooms

Naturally occurring isotopes of radon (²²²Rn) and radium isotopes (^{223,224,226,228}Ra) were used as tracers to assess submarine groundwater discharge (SGD) into Little Lagoon, AL (USA), a site of recurring harmful algal blooms (HABs). The radium isotopic data suggests that there are two groundwater sources of these tracers to the lagoon, a shallow (A1) and deeper (A2) aquifer. We estimated the fraction of each source via a three-end-member mixing model consisting of Gulf of Mexico seawater, shallow and deep groundwater. The estimated lagoonwide SGD rates based on a radium mass balance and the mixing model were 1.22?±?0.53 and 1.59?±?0.20 m³ s^-1 for the shallow and deep groundwater discharges, respectively. To investigate temporal variations in SGD, we performed several radon surveys from 2010 through 2012, a period of generally declining groundwater levels due to a drought in the southeastern USA. The total SGD rates based on a radon mass balance approach were found to vary from 0.60 to 2.87 m³ s^-1. We observed well-defined relationships between nutrients and chlorophyll-a in lagoon waters during a period when there was an intense diatom bloom in April 2010 and when no bloom existed in March 2011. A good correlation was also found between radium (groundwater-derived) and nutrients during the April 2010 period, while there was no clear relationship between the same parameters in March 2011. Based on multivariate analysis of chemical and environmental factors, we suggest that nutrient-rich inputs during high SGD may be a significant driver of algal blooms, but during low SGD periods, multiple drivers are responsible for the occurrence of algal blooms.     

Determining Spatial and Temporal Inputs of Freshwater,Including Submarine Groundwater Discharge,to a Subtropical Estuary Using Geochemical Tracers,Biscayne Bay,South Florida

Geochemical mixing models were used to decipher the dominant source of freshwater (rainfall, canal discharge, or groundwater discharge) to Biscayne Bay, an estuary in south Florida. Discrete samples of precipitation, canal water, groundwater, and bay surface water were collected monthly for 2 years and analyzed for salinity, stable isotopes of oxygen and hydrogen, and Sr²⁺/Ca²⁺ concentrations. These geochemical tracers were used in three separate mixing models and then combined to trace the magnitude and timing of the freshwater inputs to the estuary. Fresh groundwater had an isotopic signature (δ ¹⁸O = −2.66‰, δD −7.60‰) similar to rainfall (δ ¹⁸O = −2.86‰, δD = −4.78‰). Canal water had a heavy isotopic signature (δ ¹⁸O = −0.46‰, δD = −2.48‰) due to evaporation. This made it possible to use stable isotopes of oxygen and hydrogen to separate canal water from precipitation and groundwater as a source of freshwater into the bay. A second model using Sr²⁺/Ca²⁺ ratios was developed to discern fresh groundwater inputs from precipitation inputs. Groundwater had a Sr²⁺/Ca²⁺ ratio of 0.07, while precipitation had a dissimilar ratio of 0.89. When combined, these models showed a freshwater input ratio of canal/precipitation/groundwater of 37%:53%:10% in the wet season and 40%:55%:5% in the dry season with an error of ±25%. For a bay-wide water budget that includes saltwater and freshwater mixing, fresh groundwater accounts for 1–2% of the total fresh and saline water input.     

Phytoplankton Interannual Variability at Cassino Beach, Southern Brazil (1992–2007), with Emphasis on the Surf Zone Diatom Asterionellopsis glacialis

A long-term study (monthly sampling, 1992 to 2007) was conducted in the surf zone of Cassino Beach, Southern Brazil, in order to detect possible natural and/or anthropogenic disturbances. Surface water temperature (6–29°C) was the only parameter with predictable seasonal variation; salinity (14–38) was inversely related to rainfall (3.1–485.2 mm month⁻¹) and low values followed extreme precipitation periods in 1997/1998 and 2002/2003 (El Ni?o years). Asterionellopsis glacialis and chlorophyll a presented high concentrations and peak frequency until 1998, when an intense mud deposition occurred with concomitant extreme rainfall. It affected the surf zone and beach, changing the hydrology and dissolved inorganic nutrient availability. Six phytoplankton species groups were recognized with distinct responses to this mud deposition. We conclude that large-scale climatic changes, like El Ni?o Southern Oscillation, in conjunction with human activities significantly altered the phytoplankton ecology of the highly dynamic Cassino Beach surf zone.     

Effects of nitrogen fertilization on soil respiration in temperate grassland in Inner Mongolia,China

Nitrogen addition to soil can play a vital role in influencing the losses of soil carbon by respiration in N-deficient terrestrial ecosystems. The aim of this study was to clarify the effects of different levels of nitrogen fertilization (HN, 200 kg N ha⁻¹ year⁻¹; MN, 100 kg N ha⁻¹ year⁻¹; LN, 50 kg N ha⁻¹ year⁻¹) on soil respiration compared with non-fertilization (CK, 0 kg N ha⁻¹ year⁻¹), from July 2007 to September 2008, in temperate grassland in Inner Mongolia, China. Results showed that N fertilization did not change the seasonal patterns of soil respiration, which were mainly controlled by soil heat-water conditions. However, N fertilization could change the relationships between soil respiration and soil temperature, and water regimes. Soil respiration dependence on soil moisture was increased by N fertilization, and the soil temperature sensitivity was similar in the treatments of HN, LN, and CK treatments (Q ₁₀ varied within 1.70–1.74) but was slightly reduced in MN treatment (Q ₁₀ = 1.63). N fertilization increased soil CO₂ emission in the order MN > HN > LN compared with the CK treatment. The positive effects reached a significant level for HN and MN (P < 0.05) and reached a marginally significant level for LN (P = 0.059 < 0.1) based on the cumulative soil respiration during the 2007 growing season after fertilization (July–September 2007). Furthermore, the differences between the three fertilization treatments and CK reached the very significant level of 0.01 on the basis of the data during the first entire year after fertilization (July 2007–June 2008). The annual total soil respiration was 53, 57, and 24% higher than in the CK plots (465 g m⁻² year⁻¹). However, the positive effects did not reach the significant level for any treatment in the 2008 growing season after the second year fertilization (July–September 2008, P > 0.05). The pairwise differences between the three N-level treatments were not significant in either year (P > 0.05).     

Carbon dioxide,water vapor,and heat fluxes over agricultural crop field in an arid oasis of Northwest China,as determined by eddy covariance

Fluxes of carbon dioxide, water vapor, and heat were measured above crop canopy using the eddy covariance method during the 2008 maize growing season, over an agricultural field within an oasis located in the middle reaches of Heihe River basin, northwest China. The values for friction velocity, the Monin–Obukhov stability parameter, and energy balance closure indicated that the eddy covariance system at this study site provided reliable flux estimates. Results from measurements showed that the mean sensible heat flux was 70 W m⁻² with a maximum value of 164 W m⁻² (May) and a minimum value of 45 W m⁻² (July) during the maize growing season. In contrast, the mean latent heat was 278 W m⁻² with a maximum value of 383 W m⁻² (July) and minimum of 101 W m⁻² (May). The mean downward soil heat flux was 55 W m⁻² with a maximum value of 127 W m⁻² (May) and minimum of 49 W m⁻² (July). The magnitude of mean daytime net CO₂ uptake was −11.50 μmol m⁻² s⁻¹ with a maximum value of −28.32 μmol m⁻² s⁻¹ (18 and 19 July) and a minimum values of −0.32 μmol m⁻² s⁻¹ (18 and 19 May). Correlation was observed between daytime half-hourly carbon dioxide flux and canopy conductance. In addition, the relationship between carbon dioxide flux and photosynthetically active radiation for selected days during different stages of maize growing season indicated the carbon dioxide flux uptake by the canopy was controlled by actual photosynthetic activity related to the variation of green leaf area index for the different growing stages.     

Oxygen and hydrogen isotopes for the characteristics of groundwater recharge: a case study from the Chih-Pen Creek basin,Taiwan

Assessing the seasonal variation of groundwater recharge is important for effective management of groundwater resources. Stable isotopes of oxygen and hydrogen were used to estimate the sources of groundwater and seasonal contributions of precipitation to groundwater recharge in Chih-Pen Creek basin of eastern Taiwan. Based on the isotopes of precipitation (n = 177), two different local meteoric water regression lines (LMWL) can be obtained for the different seasons: δD = 8.06¹⁸O + 10.08 for wet season precipitation (May through October) and δD = 8.65δ¹⁸O + 17.09 for dry season precipitation (November through April). The slope and intercept of regression line for wet season precipitation are virtually identical to the global meteoric water line (GMWL) of Craig (1961). In contrast to during dry season precipitation due to evaporation effect the intercept of 17.09 is much higher than of the GMWL of 10. The results show the stable isotopes compositions of precipitation decrease with increasing rainfall amount and air temperature, due to the amount effect of precipitation is pronounced. The amount effect is clearly but do not show the temperature effect from January to December 2007. Using a mass-balance equation, a comparison of deuterium excess or d values of precipitation and groundwater indicates the groundwater consist of 76% wet season precipitation and 24% dry season precipitation, representing a distinct seasonal variation of groundwater recharge in study area. About 79% of the groundwater is recharged from the river water of the mountain watershed and 21% is from the rain that falls on the basin.     

Glacial runoff characteristics of the Koxkar Glacier,Tuomuer-Khan Tengri Mountain Ranges,China

This paper presents the glacial runoff characteristics of the Koxkar Glacier, China using flow records collected near the snout of the glacier in four consecutive years (2005–2008). The mean annual discharge of the Koxkar Glacier is 102.86 × 10⁶ m³, in which 93.6% occurs in ablation season (May–October). August brings about maximum discharge of about 29.6% to the total streamflow followed by July of some 26.0%. During the study years, total discharge varies little from year to year, whereas the inter-annual variability in monthly discharge is prominent, particularly in the beginning and in the end of the melt season. Seasonal runoff variability shows great monthly variations in discharge with mean coefficient of variation ranging from 0.02 in January to 0.77 in April. The mean diurnal amplitudes are found to be 0.90, 1.86, 4.71, 4.92, 1.17 and 0.43 m³ s⁻¹ for May, June, July, August, September and October, respectively. In the melt season, the maximum runoff is observed during 1800–0200 hours and the minimum occurs during 0700–1000 hours. Delaying effects are prominent in discharge over the ablation period. The time-lag between meltwater generation and its appearance in the streamflow near the snout of the glacier varies between 4.00 and 10.00 h, and time to peak varies between 10.00 and 17.00 h over the entire melt season. The relationship between discharge and temperature on monthly scale (R ² = 0.77) is better than that on daily scale (R ² = 0.55).     

Phytoplankton Size and Taxonomic Composition in a Temperate Estuary Influenced by Monsoon

Temporal and spatial variations in phytoplankton in Asan Bay, a temperate estuary under the influence of monsoon, were investigated over an annual cycle (2004). Phytoplankton blooms started in February (>20 μg chl l⁻¹) and continued until April (>13 μg chl l⁻¹) during the dry season, especially in upstream regions. The percentage contribution of large phytoplankton (micro-sized) was high (78–95%) during the blooms, and diatoms such as Skeletonema costatum and Thalassiosira spp. were dominant. The precipitation and freshwater discharge from embankments peaked and supplied nutrients into the bay during the monsoon event, especially in July. Species that favor freshwater, such as Oscillatoria spp. (cyanobacteria), dominated during the monsoon period. The phytoplankton biomass was minimal in this season despite nutrient concentrations that were relatively sufficient (enriched), and this pattern differed from that in tropical estuaries affected by monsoon and in temperate estuaries where phytoplankton respond to nutrient inputs during wet seasons. The flushing time estimated from the salinity was shorter than the doubling time in Asan Bay, which suggests that exports of phytoplankton maximized by high discharge directly from embankments differentiate this bay from other estuaries in temperate and tropical regions. This implies that the change in physical properties, especially in the freshwater discharge rates, has mainly been a regulator of phytoplankton dynamics since the construction of embankments in Asan Bay.     

Variations of Hydrodynamics and Submarine Groundwater Discharge in the Yellow River Estuary Under the Influence of the Water-Sediment Regulation Scheme

The “Water-Sediment Regulation Scheme” (WSRS) is critically important to the hydrologic evaluation of the Yellow River estuary since a huge pulse of water and sediment are delivered into the sea during a short period. We used the natural geochemical tracers radium (²²³Ra, ²²⁴Ra, ²²⁶Ra) and radon (²²²Rn) isotopes as well as other hydrological parameters to investigate the mixing variations and submarine groundwater discharge (SGD) in the Yellow River estuary under the influence of the 2013 WSRS. Dramatically elevated radium and radon isotopic activities were observed during this WSRS compared with activities measured during a non-WSRS period. Radium “water ages” indicated that the offshore transport rate nearly tripled when the river discharge increased from 400 to 3400 m³/s. We calculated the SGD flux in the Yellow River estuary based on a radium mass balance model as well as radium and radon time-series models. The SGD flux was estimated at 0.02~0.20 m/day during a non-WSRS period and 0.67~1.22 m/day during the 2013 WSRS period. The results also indicate that large river discharge tends to lead more intense SGD along the river channel direction with a large amount of fresh SGD.     

Influence of hydrogeochemical processes on temporal changes in groundwater quality in a part of Nalgonda district, Andhra Pradesh, India

Geochemical processes that take place in the aquifer have played a major role in spatial and temporal variations of groundwater quality. This study was carried out with an objective of identifying the hydrogeochemical processes that controls the groundwater quality in a weathered hard rock aquifer in a part of Nalgonda district, Andhra Pradesh, India. Groundwater samples were collected from 45 wells once every 2 months from March 2008 to September 2009. Chemical parameters of groundwater such as groundwater level, EC and pH were measured insitu. The major ion concentrations such as Ca²⁺, Mg²⁺, Na⁺, K⁺, Cl⁻, and SO₄ ²⁻ were analyzed using ion chromatograph. CO₃ ⁻ and HCO₃ ⁻ concentration was determined by acid–base titration. The abundance of major cation concentration in groundwater is as Na⁺ > Ca²⁺ > Mg²⁺ > K⁺ while that of anions is HCO₃ ⁻ > SO₄ ²⁻ > Cl⁻ > CO₃ ⁻. Ca–HCO₃, Na–Cl, Ca–Na–HCO₃ and Ca–Mg–Cl are the dominant groundwater types in this area. Relation between temporal variation in groundwater level and saturation index of minerals reveals the evaporation process. The ion-exchange process controls the concentration of ions such as calcium, magnesium and sodium. The ionic ratio of Ca/Mg explains the contribution of calcite and dolomite to groundwater. In general, the geochemical processes and temporal variation of groundwater in this area are influenced by evaporation processes, ion exchange and dissolution of minerals.     

Variation and correlations of selected heavy metals in sediment and aquatic plants in Tasik Chini,Malaysia

This paper aims to determine the correlation between Cd, Cu and Pb concentration in the sediment and in five aquatic plants sampled during wet, normal and dry seasons. Analyses of the sediment showed that concentrations of exchangeable Cd and Cd after acid reduction were higher during the wet season (October) than on other sampling dates with mean values of 0.18 and 0.29 μg g⁻¹, respectively. The concentration of Cu in the organic oxidation phase was higher in the normal season (January) than on other sampling dates with a mean value of 11.1 μg g⁻¹. The concentrations of exchangeable Pb and that in the residual phase were higher during the wet season than on other sampling dates with mean values of 1.05 and 9.18 μg g⁻¹, respectively. Cd and Pb concentrations in the leaves, stems and roots varied between sampling dates with a reduced concentration during dry season (July) and the highest metal concentrations (Cd and Pb) during wet season. There were positive correlations between Cd and Cu concentrations in the plant tissues (leaves, stems and roots) of most aquatic plants in the acid reduction fraction. Conversely, there was no positive correlation between Pb concentration in the plant tissues of all aquatic plants and the acid reduction fraction of the sediment.     

Soil salinity and sodicity in a shrimp farming coastal area of the Mekong Delta,Vietnam

Soil salinity and sodicity are environmental problems in the shrimp farming areas of the Cai Nuoc district, Ca Mau province, Vietnam. In 2000, farmers in the district switched en masse from rice cropping to shrimp culture. Due to recent failure in shrimp farming, many farmers wish to revert to a rotational system with rice in the wet season and shrimps in the dry season. So far, all their attempts to grow rice have failed. To assess soil salinity and sodicity, 25 boreholes in shrimp ponds were analysed in four consecutive seasons from 2002 to 2004. The results showed that soil salinity was quite serious (mean EC_e 29.25 dS m⁻¹), particularly in the dry season (mean EC_e 33.44 dS m⁻¹). In the wet season, significant amounts of salts still remained in the soil (mean EC_e 24.65 dS m⁻¹) and the highest soil salinity levels were found near the sea. Soil sodicity is also a problem in the district (exchangeable sodium percentage range 9.63–72.07%). Sodicity is mainly a phenomenon of topsoils and of soils near the sea. Both soil salinity and sodicity are regulated by seasonal rainfall patterns. They could together result in disastrous soil degradation in the Cai Nuoc district.     

Nitrate contamination of groundwater in an agroecosystem in Zhangye Oasis,Northwest China

In order to assess the extent of groundwater contamination by nitrate (NO₃ ⁻–N) and to provide information about the deterioration of the groundwater quality in Zhangye Oasis, Northwest China, a study was conducted in this area. The mean value of NO₃ ⁻–N concentrations in groundwater samples was 10.66 ± 0.19 mg l⁻¹. NO₃ ⁻–N concentrations exceeding 10 mg l⁻¹ (the threshold for drinking water set by the World Health Organization) were found in 32.4% of 71 wells, and were 13, 33.3, 52.4 and 50.0% in the groundwater samples from drinking wells, irrigation wells, hand-pumping wells and groundwater table observation wells, respectively. The result showed that the groundwater samples that had NO₃ ⁻–N concentrations exceeding the threshold for drinking water were mostly collected from a depth of less than 20 m. Groundwater NO₃ ⁻–N concentrations in areas used for the cultivation of vegetables, seed maize and intercropped maize were significantly higher than those in urban or paddy areas. NO₃ ⁻–N contamination of groundwater in areas with sandy soil was more severe than in those with loam soil.     

Hydrogeochemical processes in the groundwater environment of Heihe River Basin,northwest China

The Heihe River Basin is a typical arid inland river basin for examining stress on groundwater resources in northwest China. The basin is composed of large volumes of unconsolidated Quaternary sediments of widely differing grain size, and during the past half century, rapid socio-economic development has created an increased demand for groundwater resources. Understanding the hydrogeochemical processes of groundwater and water quality is important for sustainable development and effective management of groundwater resources in the Heihe River basin. To this end, a total of 30 representative groundwater samples were collected from different wells to monitor the water chemistry of various ions and its quality for irrigation. Chemical analysis shows that water presents a large spatial variability of chemical facies (SO₄ ²⁻–HCO₃⁻, SO₄ ²⁻–Cl⁻, and Cl⁻–SO₄ ²⁻) as groundwater flow from recharge area to discharge area. The ionic ratio indicates positive correlation between the flowing pairs of parameters: Cl⁻ and Na⁺(r = 0.95), SO₄ ²⁻ and Na⁺ (r = 0.84), HCO₃ ⁻ and Mg²⁺(r = 0.86), and SO₄ ²⁻ and Ca²⁺ (r = 0.91). Dissolution of minerals, such as halite, gypsum, dolomite, silicate, and Mirabilite (Na₂SO₄·10H₂O) in the sediments results in the Cl⁻, SO₄ ²⁻, HCO₃ ⁻, Na⁺, Ca²⁺ and Mg²⁺ content in the groundwater. Other reactions, such as evaporation, ion exchange, and deposition also influence the water composition. The suitability of the groundwater for irrigation was assessed based on the US Salinity Laboratory salinity classification and the Wilcox diagram. The results show that most of the groundwater samples are suitable for irrigation uses barring a few locations in the dessert region in the northern sub-basin.     

Evaluation of groundwater quality with emphasis on fluoride concentration in Nalbari district,Assam, Northeast India

This study was carried out to analyze groundwater quality in selected villages of Nalbari district, Assam, India, where groundwater is the main source of drinking water. 40 groundwater samples collected from hand pumps and analyzed for pH, EC, TDS, Ca²⁺, Mg²⁺, Na⁺, K⁺, HCO₃ ⁻, SO₄ ²⁻, Cl⁻ and F⁻. Chemical analysis of the groundwater showed that mean concentration of cations in (mg/L) is in the order Ca²⁺ > Mg²⁺ > Na⁺ > K⁺ while for anions it is HCO₃ ⁻ > Cl⁻ > SO₄ ²⁻ > F⁻. Fluoride concentration was recorded in the range of 0.02–1.56 mg/L. As per the desirable and maximum permissible limits for fluoride in drinking water recommended by WHO and by Bureau of Indian Standards (BIS), which is 1.5 mg/L, the groundwater of about 97% of the samples were found to be suitable for drinking purpose. The suitability of the groundwater for irrigation purpose was investigated by some determining factors such as sodium adsorption ratio, soluble sodium percentage, Kelly’s ratio and electrical conductivity. The value of the sodium absorption ratio and electrical conductivity of the groundwater samples were plotted in the US Salinity laboratory diagram for irrigation water. Most of the groundwater samples fall in the field of C2S1 and C3S1 indicating medium to high salinity and low sodium water, which can be used for irrigation on almost all types of soil with little doubt of exchangeable sodium. The hydrochemical facies shows that the groundwater is Ca-HCO₃ type.     

Phytoplankton Biomass and Production in Subtropical Hong Kong Waters: Influence of the Pearl River Outflow

The size-fractionated phytoplankton biomass and primary production were investigated in four contrasting areas of Hong Kong waters in 2006. Phytoplankton biomass and production varied seasonally in response to the influence of the Pearl River discharge. In the dry season, the phytoplankton biomass and production were low (<42 mg chl m⁻² and <1.8 g C m⁻² day⁻¹) in all four areas, due to low temperatures and dilution and reduced light availability due to strong vertical mixing. In contrast, in the wet season, in the river-impacted western areas, the phytoplankton biomass and production increased greater than five-fold compared to the dry season, especially in summer. In summer, algal biomass was 15-fold higher than in winter, and the mean integrated primary productivity (IPP) was 9 g C m⁻² day⁻¹ in southern waters due to strong stratification, high temperatures, light availability, and nutrient input from the Pearl River estuary. However, in the highly flushed western waters, chl a and IPP were lower (<30 mg m⁻² and 4 g C m⁻² day⁻¹, respectively) due to dilution. The maximal algal biomass and primary production occurred in southern waters with strong stratification and less flushing. Spring blooms (>10 μg chl a L⁻¹) rarely occurred despite the high chl-specific photosynthetic rate (mostly >10 μg C μg chl a ⁻¹ day⁻¹) as the accumulation of algal biomass was restricted by active physical processes (e.g., strong vertical mixing and freshwater dilution). Phytoplankton biomass and production were mostly dominated by the >5-μm size fraction all year except in eastern waters during spring and mostly composed of fast-growing chain-forming diatoms. In the stratified southern waters in summer, the largest algal blooms occurred in part due to high nutrient inputs from the Pearl River estuary.     

Hydrochemical characteristics of groundwater in the Zhangye Basin, Northwestern China

The Zhangye Basin, located in arid northwest China, is an important agricultural and industrial center. In recent years rapid development has created an increased demand for water, which is increasingly being fulfilled by groundwater abstraction. Detailed knowledge of the geochemical evolution of groundwater and water quality can enhance understanding of the hydrochemical system, promoting sustainable development and effective management of groundwater resources. To this end, a hydrochemical investigation was conducted in the Zhangye Basin. Types of shallow groundwater in the Zhangye Basin were found to be HCO₃ ⁻, HCO₃ ⁻–SO₄ ²⁻, SO₄ ²⁻–HCO₃ ⁻, SO₄ ²⁻–Cl⁻, Cl⁻–SO₄ ²⁻ and Cl⁻ . The deep aquifer groundwater type was found to be HCO₃⁻–SO₄²⁻ throughout the entire area. Ionic ratio and saturation index calculations suggest that silicate rock weathering and evaporation deposition are the main processes that determine the ionic composition in the study area. The suitability of the groundwater for irrigation was assessed based on the US Salinity Laboratory salinity classification and the Wilcox diagram. In the study area, the compositions of the stable isotopes δ¹⁸O and δD in groundwater samples were found to range from −4.00 to −9.28‰ and from −34.0 to −65.0‰, respectively. These values indicate that precipitation is the main recharge source for the groundwater system; some local values indicate high levels of evaporation. Tritium analysis was used to estimate the ages of the different groundwaters; the tritium values of the groundwater samples varied from 3.13 to 36.62 TU. The age of the groundwater at depths of less than 30 m is about 5–10 years. The age of the groundwater at depths of 30–50 m is about 10–23 years. The age of the groundwater at depths of 50–100 m is about 12–29 years. For groundwater samples at depths of greater than 100 m, the renewal time is about 40 years.