A long-term experimental study of the reactivity of basement rock with highly alkaline cement waters: reactions over the first 15 months

A series of long-term laboratory experiments was started in 1995 to investigate longer-term dissolution/precipitation reactions that may occur in the alkaline disturbed zone surrounding a cementitious repository for radioactive waste. They consist of samples of UK basement rock reacting with either Na-K-Ca-OH water (‘young’ cement porewater) or Ca-OH water (‘evolved’ cement porewater) at 70°C. This paper summarizes results of reactions occurring over the first 15 months. Experiments of both fluid types showed many similar features, though primary mineral dissolution and secondary mineral precipitation were more extensive in the experiments involving Na-K-Ca (younger) cement porefluids compared to more evolved (Ca-rich) cement porefluids. Dissolution of dolomite, and to a lesser extent silicates (probably K-feldspar, but also possibly mica) occurred relatively rapidly at 70°C. Dolomite dissolution may have been a key factor in reducing pH values, and may be a key mineral in controlling the extent of alkaline disturbed zones. Dissolution was followed by precipitation of brucite close to dolomite grains, at least two generations of C-S-H phases (which may have contained variable amounts of K, Al and Mg); overgrowths of calcite; small crystals of hydroxyapophyllite; and elongate crystals of celestite. Though hydroxyapophyllite was observed (a phase commonly associated with zeolites), there was no evidence for the formation of zeolites in the experiments. Fluid chemical changes track the mineralogical changes, with C-S-H phases being a major control on fluid chemistry. In the ‘young’ porewater experiments there were decreases in pH, and K, Ca and Mg concentrations, together with transitory increases in SiO2 concentrations. In the ‘evolved’ porewater experiments there were decreases in pH, Mg, Ca and Sr concentrations, together with small increases in K and SiO2 concentrations. A number of experiments are still running, and will be sampled in coming years

Rock alteration at high pH relevant to the geological disposal of radioactive waste

Dissolution of the cement used in radioactive waste repositories will produce a high pH leachate (pH 10.5-13.1) that will evolve in pH and composition over time, remaining hyperalkaline for 106 years. This will migrate into the repository host rock potentially altering the rock’s physical and chemical properties, and its function as a barrier to radionuclide migration. To investigate the possible alteration over timescales relevant for geological disposal (104 to 105 years), previous studies included short term (< 2 years) laboratory experiments, natural analogue studies (100s to 105 years) and predictive modelling. However, the geochemical and mineralogical processes which may occur as such systems evolve remain poorly constrained. Here, rock alteration in high pH solutions, analogous to cement leachates, has been investigated on short (1 year), medium (15 years) and long (100s years) timescales. The short term investigation agreed with previous studies finding that silicate mineral dissolution and C-S-H precipitation were the predominant alteration processes. In the medium term study, although initially silicate mineral dissolution and C-S-H precipitation occurred, by 15 years of reaction neo-formed Mg-silicates replaced the C-S-H. To investigate rock alteration on a longer timescale Herbert’s Quarry, where high pH fluids have existed for 100s years, was characterised and assessed as a potential analogue site. However, the site was found to have limited applicability in the context of the high pH rock alteration expected at a radioactive waste repository. The impact of rock alteration on U(VI) behaviour was also investigated in the short and medium term. In the short term, U(VI) behaviour during rock alteration varied between solutions representative of different periods of cement leachate evolution, with potential impacts for radionuclide transport. The sorption of U(VI) to unaltered rock and rock altered for 15 years was also investigated. This suggested alteration may increase a rock’s sorption capacity for U(VI)

Alluvial aquifers in the Mzingwane catchment: Their distribution, properties, current usage and potential expansion

The Mzingwane River is a sand filled channel, with extensive alluvial aquifers distributed along its banks and bed in the lower catchment. LandSat TM imagery was used to identify alluvial deposits for potential groundwater resources for irrigation development. On the false colour composite band 3, band 4 and band 5 (FCC 345) the alluvial deposits stand out as white and dense actively growing vegetation stands out as green making it possible to mark out the lateral extent of the saturated alluvial plain deposits using the riverine fringe and vegetation . The alluvial aquifers form ribbon shaped aquifers extending along the channel and reaching over 20 km in length in some localities and are enhanced at lithological boundaries. These alluvial aquifers extend laterally outside the active channel, and individual alluvial aquifers have been measured with area ranging from 45 ha to 723 ha in the channels and 75 ha to 2196 ha on the plains. The alluvial aquifers are more pronounced in the Lower Mzingwane, where the slopes are gentler and allow for more sediment accumulation. Estimated water resources potential ranges between 175,000 m3 and 5,430,000 m3 in the channels and between 80,000 m3 and 6,920,000 m3 in the plains. Such a water resource potential can support irrigation ranging from 18 ha to 543 ha for channels alluvial aquifers and 8 ha to 692 ha for plain alluvial aquifers. Currently, some of these aquifers are being used to provide water for domestic use, livestock watering and dip tanks, commercial irrigation and market gardening. The water quality of the aquifers in general is fairly good due to regular recharge and flushing out of the aquifers by annual river flows and floodwater. Water salinity was found to increase significantly in the end of the dry season, and this effect was more pronounced in water abstracted from wells on the alluvial plains. During drought years, recharge is expected to be less and if the drought is extended water levels in the aquifers may drop substantially, increasing salinity problem

Influence of neighbourhood socioeconomic position on the transition to type II diabetes in older Mexican Americans: the Sacramento Area Longitudinal Study on Aging

To examine the influence of neighbourhood socioeconomic position (NSEP) on development of diabetes over time

The association of parental and offspring educational attainment with systolic blood pressure, fasting blood glucose and waist circumference in Latino adults

Objective: The objective of the study is to evaluate the association of intergenerational educational attainment with cardiovascular disease (CVD) risk factors among US Latinos. Methods: We used cross-sectional data from the Niños Lifestyle and Diabetes Study, an offspring cohort of middle-aged Mexican-Americans whose parents participated in the Sacramento Latino Study on Aging. We collected educational attainment, demographic and health behaviours and measured systolic blood pressure (SBP), fasting glucose and waist circumference. We evaluated the association of parental, offspring and a combined parent–offspring education variable with each CVD risk factor using multivariable regression. Results: Higher parental education was associated only with smaller offspring waist circumference. In contrast, higher offspring education was associated with lower SBP, fasting glucose and smaller waist circumference. Adjustment for parental health behaviours modestly attenuated these offspring associations, whereas adjustment for offspring health behaviours and income attenuated the associations of offspring education with offspring SBP and fasting glucose but not smaller waist circumference, even among offspring with low parental education. Conclusions: Higher offspring education is associated with lower levels of CVD risk factors in adulthood, despite intergenerational exposure to low parental education