Hidrogeología e Hidrogeoquímica de los acuiferos del Bajo Almanzora

Resum

To study the relationships between aquifers in preorogenic materials and sedimentary filling, the quality of their waters and the processes that affect those, I've selected a study area located in the Bajo Almanzora region. In this region the suitability of water, in a quantitative and qualitative point of view, is one of the principal factors conditioning the economical and agricultural development. Hydrogeochemical processes determine the chemical quality of the water bodies. That processes are associated to chemical equilibria and are conditioned by climatological, hydrological and geological factors. In the study of the precipitation, I've taken the reference period of 1961-2000 to calculate the characteristic parameters of the pluviometric series. The study of the spatial distribution and temporal distribution of precipitations permit to acknowledge directional patterns and cycles. To calculate Usefull Rain a Thornthwaite Balance was perfomed, using daily precipitation data. In the study of the hydrology, work has been done with MDTs and the distribution of the river gauges data, deducting estimators for dry, medium, and humid year. In addition to the water runoff another waters from external basins come by the Negratin and Tajo Segura Sur transfers. The desalination plant of Palomares and of Carboneras supply to this area, too. The Betic rocks of Paleozoic-Triassic age form the Preorogenic socle, where it's deposited the sedimentary filling that fills in the depressions and the basins. The lithology of the sediments that fill in the basins during the upper Miocene and the Pliocene is composed of conglomeratic and calcarenitic facies in basin borders, that evolve to marly facies in the inner part of the basin. In the sedimentary filling five hydrogeologic units exist: El Saltador, Pulpí, Overa, Bajo Almanzora and La Ballabona. Another minor aquifers of inter-granular porosity little cemented, are the existing in Llanos del Chaupí and Guazamara corridor. The intensive exploiting of the detrital aquifers generated important conoids in the freatical sheets. To the end of the 80s a period of slow recovering was entered in some water bodies. The aquifers in Preorogenic materials are enhanced by their high topographical position, locating in areas favored to recharge. The study of the spatial and temporal variability of the electrical conductivity permit acknowledges the importance of dissolution processes from alluvial sediments of ramblas and torrents. The notable change of the exit temperature in waters is usually related to changes in entering and exiting intermitting or occasional fluxes. Points of thermal nature, with a temperature of exit over 19ºC are observed. In the study of the concentrations of major components in waters of the different aquifers, the use of box-plot diagrams makes easier identifying extreme values. The seasonal changes are observed easily by the joint representation of alkalinity and C.E. for the different samplings. Some points have changed of typology depending of the sampling, finding bicarbonated calcic waters in deep boreholes. The spatial distribution of the different typologies are studied by the representation of Stiff diagrams included in a map. The relations between physical-chemical variables and major components can be studied easily from the correlation matrix. The formation of synthetic variables independent between them, by factor analysis of physical-chemical variables and major components, point to the differentiation of hydrogeochemical processes. The differentiation of groups of samples by a objective criteria has been done applying cluster analysis of Q-mode. The variations in chemistry are acknowledged by the study of ionic ratios as rCl-/rCO3H-, rSO42-/rCl-, rMg2+/rCa2+, (rCa2+ + rMg2+)/rCl- y (rNa++ rK+)/rCl- and the index (rCa2+ × (rCO3H-)2)1/3. These variables are found related to lithology and to the existence of fluxes of CO2 from the deep Preorogenic rocks. The dpH and the equilibrating pCO2 result indicators of agressive properties of the waters. Bromide, fluoride, silica, iron, manganese, lithium, strontium and boro constitute minor components that occasionally overcomes 1 mg/L concentration. In the occurrence of the minor components iron plays an important role maintaining an inverse relation with Eh. The hydroxides of iron show adsorbent properties controlling the release/dissolution of trace metals. The processes of saline supply have been recognized by the use of the pCl-/pBr- ratio, observing the leaking of waste waters and irrigation waters. To use a chemical criterion on the thermal or non-thermal characterization of the waters I've used the pB/pLi+ ratio. I've represented the quantitative variation of the minor components in frequency hystograms. Aluminum, related to feldspars; cobalt, in the surroundings of deep fluxes; niquel, related to carbonates; zinc related to deep fluxes and carbonates, too; selenium related to sulphates; lanthanides forming complexes with carbonates or lead forming saline complexes are some trace elements that support new evidences on hydrological cycle and hydrogeochemical issues. Selecting 22 variables for the factor analysis with minor and trace elements, from the group that presented higher number of data I've recognised 6 factors; The first is related to movement of cations at acid pH and reducing environment and the second to movement of bivalent cations (Zn, Ca, Mn). The cluster analysis permitted to differentiate 7 groups attending their composition on trace and minor elements. The stable isotopic data of water are adjusted to a meteoric line, with different lines of evaporation. The results indicate dominance of the influence of Atlantic origin over the influence of Mediterranean origin. To recognise the age of underground waters and recycling indices in the aquifer, Tritium analysis were made. To establish hypothesis on the origin of C analysis of 13C were performed on CIT. Two samples (deep boreholes) are found near the range of CO2 of mantle origin. Analysis of 34SSO4 were performed with the objective of studying the origin of sulphates and the possible existence of sulphate reduction phenomena. The dedolomitization process is almost constant by all the path of waters. The use of the hydrogeochemical modelling helps to determine the occurrence and quantify hydrogeochemical processes that contribute to water salinization. The water of the Negratín transfer and the desalination plants of Palomares and Carboneras, recently incorporated to management may change the underground water quality, leaking to the aquifers. The accomplishment of uploaded waters in the area to the potability laws is examined, the same as limits and utilization rules for irrigation practices. The degree of accomplishment of Good Agriculture Practices, relating to water uses, is examined, too. At last, Principal problems of pollution considered as risky to aquifers are pointed.