Recarga a los acuíferos españoles mediante balance hidrogeoquímico
Authors:   , Alcalá García Francisco Javier 
Published: July 2023

Esta tesis doctoral sienta las bases para calcular la recarga por la lluvia a los acuíferos por balance del ion cloruro (CMB), estimando en régimen estacionario y discutiendo su valor distribuido para todo el territorio español. Se aporta un valor de la tasa de recarga por la lluvia al terreno esperable en un lugar. El estudio de las variables del CMB es una contribución principal de este trabajo. La tasa media de deposición total de Cl en el centro peninsular es de 0,2 a 0,5 g·m-2·a-1. Es de 1 a 30 g·m-2·a-1 en la zona costera atlántica y de 1 a 15 g·m-2·a-1 en la mediterránea; en ambos casos con un fuerte gradiente de entre 0,1 y 1 g·m-2·a-1·km-1. Similar aspecto se produce en las áreas insulares. El coeficiente de variación de la tasa de deposición total de Cl aumenta de 0,10-0,20 en las zonas costeras hasta más de 0,40 en el interior peninsular; también aumenta con la cota topográfica. Este comportamiento se reproduce en los territorios insulares. El uso de la relación molar R = Cl/Br y ciertos criterios hidroquímicos de selección de muestras han asegurado que el contenido en Cl del agua de recarga no afectada en su salinidad natural deriva esencialmente del aerosol marino. Éste es de 10 a 50 mg/L en la zona costera atlántica y de 25 a 100 mg/L en la mediterránea. En el interior peninsular se miden contenidos entre 5 y 20 mg/L, e inferiores a 1 mg/L en zonas de cumbres. Los gradientes tentativos son de 0,2 a 1 mg·L-1·km-1 en zonas costeras. La situación en ambos archipiélagos es similar, aunque con contenidos en Cl y gradientes mayores. El coeficiente de variación del contenido en Cl es de 0,05-0,25 en las zonas costeras y de 0,15 a 0,55 en el interior peninsular. En los archipiélagos, el CV varía entre 0,10 y 0,20 en las zonas costeras de todas las islas y entre 0,10 y 0,40 en las islas de menor relieve. El flujo de Cl por la escorrentía directa, AE, oscila entre 2 y 8 g·m-2·a-1 en zonas costeras, y entre 0,02 a 0,05 g·m-2·a-1 en zonas de interior. El CVAE varía entre 0,10 y 0,30 en zonas costeras, y entre 0,40 y 1 en el interior. Los gradientes en la costa varían entre 0,05 y 0,20 g·m-2·a-1·km-1, y son de ~ 0,01 g·m-2·a-1·km-1 en el interior. La situación es similar en ambos archipiélagos. Las variables del CMB son características de un lugar y se han regionalizado espacialmente mediante krigeado ordinario (KO) e inverso de la distancia (ID), utilizando una malla regular de 5113 celdas de 10 km que cubre todo el territorio español. En cada nodo se ha interpolado un valor medio y un valor de coeficiente de variación de cada variable para obtener en ese nodo un valor de recarga media anual y un valor de su variabilidad natural. Los datos de las variables del CMB se han ajustado mediante semivariogramas teóricos de tipo esférico. Los valores medios de las variables se estabilizan entre 88 y 120 km; rango similar al obtenido para otras variables hidrológicas estimadas a la misma escala y para periodos plurianuales. Los coeficientes de variación se estabilizan entre 11 y 70 km. La varianza de estimación del contenido en Cl del agua de recarga es inferior al valor medio estimado en zonas costeras y de interior, y suele ser mayor al valor medio estimado para las otras dos variables en zonas costeras y de interior. Ambos métodos dan un rango de recarga media anual entre 1 y 800 mm·a-1 y estiman de forma similar valores entre 100 y 300 mm·a-1 y superiores. Los valores inferiores a 100 mm·a-1 están mejor estimados por el ID. Los mayores porcentajes de recarga son de ~ 60 % de la lluvia y se producen en zonas calcáreas peninsulares. Los porcentajes más bajos se miden en terrenos detríticos, en zonas semiáridas peninsulares e insulares y en ciertas zonas de interior donde afloran materiales cristalinos. El coeficiente de variación de la recarga oscila para el ID entre 0,15 y 1,7, y para el KO entre 0,2 y 1,7. Se recomienda usar el mapa de recarga media y de su coeficiente de variación obtenido mediante ID. La estimación de la recarga al terreno, sea éste acuífero o no, mediante el CMB usando métodos geoestadísticos de interpolación espacial de datos puede considerarse satisfactoria en la mayor parte del territorio español; es algo incierta en zonas costeras, de alta variación orográfica o con pocos datos disponibles, y es bastante acertada en zonas con escaso control orográfico, lejos de la costa y con datos iniciales disponibles.

This doctoral thesis establishes the bases for calculating the recharge by the rainfall to the aquifers through the ion chloride mass balance method (CMB). The study of the deposition rate of Cl allows estimating and discussing the distributed recharge to the land in Spain. This work provides the expectable value of recharge rate by the rainfall in a site, not the water volume stored. This last concept depends of average residence time of groundwater, which has not been studied and which is not a subject of this work.
In steady-state condition, the water-table receives a mass flow of Cl equal to rainfall contribution, after subtracting the Cl flow in runoff. Calculations are for a period of time large enough to avoid the influence of adjacent periods. The characterization of the variables of the CMB is a main contribution of this work.
The mean value of Cl bulk deposition rate in the center of the Peninsula oscillates between 0.2 and 0.5 g·m-2·yr-1, whereas in the Atlantic and Mediterranean coastal zones it varies respectively between 1 and 30 g·m-2·yr-1 and 1 and 15 g·m-2·yr-1. Tentative gradients between 0.1 and 1 g·m-2·yr-1·km-1 are estimated from the coast to the center.
Similar circumstances are observed in islands, although with higher average values and gradients. The coefficient of variation of CL bulk deposition rate increases from the coastal zones inland, passing from ∼ 0.10-0.20 in the coastal fringe to more than 0.40 in the interior; it also increases with higher altitude. This behaviour is reproduced in island territories. In the peninsula a decrease of the coefficient of variation of ~ 0.50 to ~ 0.10 from the NW coastal zone towards the interior is also observed; the values of ~ 0.40 or more measured in the mountainous reliefs of the center increase towards the SE.
Tracers and certain hydrochemical criteria have been used for the sample selection in order to assure that the fresh water content derives essentially from the marine aerosol and that its natural salinity has not been affected. The molar ratio R = Cl/Br in rain water and ion fresh water is similar in a same place, being of ~ 650 in coastal zones (close to the marine value), of 300-500 in inland areas, of 200-500 in summit areas, and less than the marine value in coastal zones affected by urban pollution. The use of R allows discarding samples with a non-atmospheric origin of Cl.
The Cl contents of fresh water varies between 10 and 50 mg/L, and 25 and 100 mg/L in the Atlantic and Mediterranean coastal zones, respectively. Cl contents between 5 and
20 mg/L are measured in the center of the Peninsula, and usually they are less than 1 in high areas of the main mountain ranges. Tentative gradients (in mg·L-1·km-1) between 0.2 and 1 are estimated in coastal zones. The situation is similar in both archipelagos, although with greater Cl contents and greater territorial gradients. The coefficient of variation of the CL fresh water content is 0.05-0.25 in coastal zones for sampling periods of 5 years or more. CV oscillates between 0.15 and 0.55 in inland areas. CV varies between 0.10 and 0.20 in all island coastal zones, and between 0.10 and 0.40 in the relatively low relief of the Canary Islands; it is somewhat smaller in the Balearic islands.
The Cl flow in runoff, AE, oscillates between 2 and 8 g·m-2·yr-1 in coastal zones, and between 0.02 and 0.05 g·m-2·yr-1 in the center of the Peninsula. CVAE varies between
0.10 and 0.30 in coastal zones, and between 0.40 and 1 in inland areas. Tentative gradients between 0.05 and 0.20 g·m-2·yr-1·km-1 are estimated in coastal zones, and decrease to ~ 0.01 in the interior. In the insular areas of great relief, there is a clear radial component of AE, that decrease from the coast towards the interior, with tentative gradients of ~ 0.5 g·m-2·yr-1·km-1 and CVAE close to 1. AE is usually half or an order of magnitude less than the bulk deposition rate of Cl measured in the same geographical place.
The variables of the CMB are characteristic of a given site and this their spatial distribution can be interpolated applying methods of spatial interpolation: ordinary kriging (OK) and inverse of the distance (ID) methods. A regular network of 5113 cells of 10 km covering all the Spanish territory has been used. In every node the mean value and the coefficient of variation of each variable have been interpolated to obtain in that node the mean value of recharge and its natural variability. The spatial estimation error of the input variables has been calculated, but not how this propagates when the numerical model of calculation is applied, in this case the equation of the CMB.
The data of the variables of the CMB have been adjusted through theoretical, spherical type semi-variograms, assuming a negligible nugget effect. The variable range stabilizes between 88 and 120 km; these values are similar values to the ones obtained for other hydrological variables estimated for all the Spanish territory for pluriannual periods. This range similarity confirms that the main spatial distribution features have been identified adequately, at least in its order of magnitude. The coefficients of variation stabilize between 11 and 70 km. The variance of estimation of fresh water Cl content is half the mean value estimated in inland and coastal areas or less. The variance of estimation of the other two variables of the CMB is similar or greater than the mean value estimated in coastal and inland zones. Both methods overestimate the low values and underestimate slightly the high values measured for all the variables; their bias is low to moderate for the fresh water Cl content and moderate to high for the other variables.
Both methods give a range of annual mean recharge to the land between 1 and
800 mm·yr-1. Both methods obtain similar results for the recharge range, between 100 and 300 mm·yr-1 or higher. The values less than 100 mm·yr-1 are better estimated for the ID. The higher rainfall recharge percentages are around 60%, with isolated values of 80%, and take place in peninsular carbonate areas. The lowest percentages are measured in some sedimentary formations, in peninsular and insular semiarid areas, and in certain inland areas where crystalline materials occur. Occasional minimum values of ~1% are measured. The recharge coefficient of variation for the ID oscillates between 0.15 and 1.7, and for the OK between 0.2 and 1.7. Is recommended to use the mean recharge and its coefficient of variation obtained through ID.
The high spatial and temporal variability of the variables makes the CMB calculated recharge uncertain in the coastal fringe, decreasing its interannual variability and increasing its space variability. Recharge evaluation in the center of the peninsula is more accurate; the coefficients of variation are greater than in the coast but more homogeneous spatially. The estimation of recharge to the land, where an aquifer may exist or not, by means of CMB and using geostatistical methods for spatial data interpolation can be considered satisfactory in most of the Spanish territory; it is something deficient in areas of high topographical variation and coastal zones with scarce data, and quite accurate in those areas with little topographical control, far from the coast and where initial data is available.