Microclimatic studies in Paranhos water galleries , Porto ( Portugal )

Paranhos constitutes one of the main water galleries excavated in the granite substratum of Porto City (Portugal) to gather the groundwater for public use during the past six centuries. The interest of this water gallery is double: a) Cultural Heritage and scientific value, b) Underground geoturism. A multidisciplinary research to study the microclimatology, geochemistry, mineralogy and geomicrobiology of the Paranhos catchwork galleries is currently in progress. The characterization of the main environmental parameters (air-rock temperature, relative humidity, CO2, Rn, etc.) in the internal atmosphere would allow the establishment of: a) Fluid-rock interactions and development of a physical-chemical model of air-water-rock, as a base to define their relations with unusual mineral formations and microbial communities b) Microclimatic parameters and determination of effective radioactive dose for visitors based on Rn studies, as potential use as underground geotourism site under safe conditions. Figure 1. A). Geographical location of the Paranhos and Salgueiros Spring galleries. Modified from Chaminé et al. (2010). B) Sketch of the studied section of the tunnel that corresponds to white square in Figure 1A, showing three microclimatic stations. a) Cultural Heritage and scientific value; aquifer vulnerability, urban speleological mapping and geo-engineering studies have been developed in the Paranhos galleries, but other scientific values can be remarked as the presence of unusual mineral formations (Sanchez-Moral et al. 2011) (e.g. Evansite) and microbial communities. Evansite comprise microlayers of amorphous hydrous aluminum-phosphate phases Al2(PO4)3-5.nH2O) together with hydrolyzed uranyl groups and hydroxide UO2(OH)2 precipitates. The microbiological study of these environments represents an approach to the discovery of the large bacterial diversity, the understanding of the role and functioning of these microorganisms and their implications on the conservation of the geo-sites or their affinity to particular hydrological processes or supply of contaminants from the external soil. Recent scientific assessment revealed that biological activity played a major role in the development of these manganese oxides, specifically birnessite and todorokite (Miller et al. 2012).Additional interest is related to the conservation issues of geo-sites. b) Underground geotourism; as economical resource to attract tourists and investment. Artificial and stable underground spaces such as aqueducts or water galleries can be suitable for underground geotourism, previous establishment of the safe conditions for visitors. The aim of the current study is the characterization of the geochemical and microclimatic dynamics of the Paranhos water gallery with special emphasis on the study of exchange cycles of atmospheric CO2. Concerning the establishment of safe conditions for potential visitors, the study also deals about characterization of Rn gas emanation from host rock granite and radioactive minerals. MATERIAL AND METHODS Three fixed microenvironmental sampling stations were established in the Paranhos gallery (Porto 1, 2 and 3, Figure 1B) allowing the continuous monitoring of next parameters of air: 1) Air temperature and relative humidity with HOBO Pro v2 and Tinytag Plus 2 data loggers; 2) CO2 concentration measured using a ELG-33 datalogger (CO2meter). This sensor is located in a permanent station (Porto 2). All parameters are registered every 1 hour and the records are downloaded every 3 months. Radon 3) Gas Rn measured by passive integration method (3 months period) with Kodalpha Radon dosimeter (GT Analytic). In order to characterize cave levels of CO2, δCO2 and CH4 several air samples are taken every six months in an established points network along the gallery. Samples were obtained by filling Tedlar air sampling bags with capacity of 1 L and analyzed afterwards in laboratory by using an optic spectrometer Picarro G2101-i. Isotopic relationship δCO2 is measured as a calculation from CO2 and CO2 concentrations analyzed by the spectral system. During the same campaign, a 24-48 hours test for radon gas is performed at Porto 2 station using a Radon gas detector Durridge, RAD7. 3. RESULTS AND DISCUSSION Monitoring period was initiated in February 2012. The 6 months period of microclimatic characterization in the Paranhos gallery is providing next data: Air temperature and relative humidity: constant relative humidity values are near saturation (100%). Porto 1 and 3 stations present high response to daily changes, but a temperate response to seasonal cycles. Porto 2 is characterized by an accumulative effect of air temperature that could be related to the higher topographic location acting as hot air trap (Fig. 2A). Figure 2. A) Temperature record in the three stations (Porto 1, 2 & 3). B) CO2 record in Porto 2 station. CO2: the low CO2 average concentration (around 700 ppm) of winter-spring period increased to 1,200 ppm during late Spring-Summer; this effect can be interpreted as an indication of the summer reduction of the ventilation in the Porto 2 section (Fig. 2B). Rn: radon gas is used as a quantitative index of ventilation in subterranean environments, but also it is fundamental for calculation of occupational dose of potential visitors. Passive Kodalpha detectors are indicating a clear seasonal increase of radon concentrations in Porto 3 from the winter-spring to summer season, and opposite pattern for Porto 2 (Table 1). A 48 hours test of continuous monitoring (30 minute data) in Porto 2 has also allowed the identification of this seasonal behavior, but also daily oscillations of coupled Rn and CO2 (Fig. 3). Table 1. Radon activity of air (Rn, Bq • m) in the three stations. _____________________________________________________________________________ Period Porto 1 Porto 2 Porto 3 _____________________________________________________________________________ 21/02/2012 07/05/2012 15992 ± 1439 18819 ± 1506 7528 ± 678 07/05/2012 26/07/2012 12401 ± 1868 14049 ± 983 9131 ± 639 26/07/2012 06/09/2012 18784 ± 1315 12749 ± 892 12951 ±907 _____________________________________________________________________________ Microenvironmental mapping from two campaigns (February and September 2012) for CO2, δCO2 and CH4 has allowed the observation of the spatial-seasonal variation of these parameters in the tunnel. This environment, as many caves, is characterized by relatively high levels of CO2 with very low δC isotopic signal values and low levels of CH4 compared with the external atmosphere. CO2 present in the internal atmosphere has a soil-derived origin. The highest CO2 concentrations, minimum CH4 and lighter isotopic values in both measured periods correspond to the Porto 2 section, although it is relatively close to the St. Dinis Entrance. This could be related to a relative isolation of the Porto 2 area respect the rest of the tunnel. Figure 3. 48 hours continuous monitoring for Rn and CO2 (Porto 2, early September 2012).