Pollution Potential of Natural Sulphurous Groundwater from the Use of Geosynthetics in Underground Works Near Mineral Water Abstractions for Medical Spas

Abstract

Geosynthetics are used in underground works, namely geotextiles for drainage and geomembranes for waterproofing. Because some groundwaters are aggressive to the materials they contact, as is the case of the sulphurous waters used in medical spas, the question arose as to whether those materials might be degraded and, in the process, contaminate the natural groundwaters. The appearance of unusual chemical elements in the waters of the medical spa is enough to be considered contaminated and therefore leads to the closure of those establishments. Once the question was raised, an experimental plan was developed to acquire some knowledge about the situation. Thus, in this chapter, after an introduction on the importance of the subject, and a brief survey on the state of the art, the geosynthetic materials studied are presented in detail, as well as the chemical composition of virgin groundwater involved in the process. The methodology implemented is presented, and the results are shown and discussed. Finally, the main conclusions on the evolution of the physical and mechanical parameters of the geosynthetics over time (8 months of study) are presented, with special focus on the chemical changes in groundwater quality when geosynthetic materials are used in contact with them.

1. Introduction

The groundwaters of the sulphurous type are very special by heaving characteristics that give therapeutic properties. For this reason, they are frequently classified as natural mineral waters with qualifications for thermalism. Thermalism is the activity that is practiced in a Medical Spa, which functions as a particular hospital where its primary medication is natural mineral water, which is used according to medical prescription, as a consequence of the user’s health problem, or eventually, used only as actions of well-being and relaxation.

The practices used in a medical spa are various types, being organized into two main groups: i) balneotherapy, with immersion techniques in the swimming pool, simple immersion bath with still water, hydromassages, shower techniques, such as Vichy massage, circular shower, and steam techniques, such as Turkish bath, hammam bath, Bertholaix, spinal steam, and foot and hand steam; and ii) ORL - Respiratory Tracts, such as nasal irrigation, aerosol, spraying and nebulization.

The diseases treated are essentially pathological rheumatic and respiratory.

Natural mineral water has a very well-defined chemical composition and keeps high stability in its chemical quality. Besides being suitable in microbiological terms, i.e., those may create health problems without pathogenic microorganisms.

To the quality and chemical stability be adequate, among several aspects, the following should be highlighted: the exploitation flow rate should never exceed the flowing torrent considered as admissible, in order not to exceed the laminar regime of the underground flow; the protection perimeter must be strictly respected, it means, the circulation of people and goods, urban occupation, construction of works, excavations, use of materials, among many others, cannot in any way compromise the rules to have complied, especially in the Immediate Protection zone of sulphurous water abstraction (the very close area around the abstraction).

In medical spas, there are often several springs of sulphurous water, which are sometimes necessary many times to drain them, which is a current situation is done using geotextiles. On the other hand, in proximity areas of the abstraction sites, it is necessary to carry out waterproofing to avoid infiltrations of surface water, having to use flow barriers, as geosynthetics geomembranes that have these functions may come into contact with sulphurous waters.

It must be emphasized that it is common knowledge that sulphurous waters are very aggressive to some materials because they quickly accelerate their degradation, even leading to the need to replace them, with significant immediate economic loss and other consequences.

This way face the presented consequences, with the tendency to use the geosynthetics in various works associated with sulphurous waters, the question raised about the durability of those materials, not only because it is essential to know about the achievement of its functions over time, but also because there is a need to know if those possible changes in the materials can put in danger the quality of the groundwater, which is required to be of very high stability since it is used as a medication, as already mentioned. In case the sulphurous water includes chemical elements that are not part of its normal quality, it will be considered contaminated, which with high probability can carry to the closure of the respective medical spa.

Studies about the durability of geosynthetics have been frequent, mainly researching the effect of ultraviolet rays, temperatures, leachate of sanitary landfill, acid, and basic solutions, among others [1, 2, 3, 4, 5, 6, 7].

Studies on the durability of geosynthetics when immersed in situations with sulphurous groundwater have been carried out by the working group of the authors of this chapter, deserving reference to the works carried out based on permeability tests of geotextiles [8], on resistance tests (punching and tensile traction) in a geomembrane and in various geotextiles [9, 10], in tearing strength tests in geotextiles [11], and in several joint situations for various geosynthetics with some conclusions on potential changes in sulphurous groundwater [12]. Studies about groundwater quality when in contact with geosynthetics are rare. However, there are some exceptions, such as studies with geotextiles to be incorporated into permeable pavements to show that their use improves the water quality that infiltrates the surface of permeable pavements [13].

2. Studied materials and methodology

The geosynthetics used in this investigation are three geotextiles and one geomembrane (Figure 1). The geotextiles are non-woven and use three different types: one geotextile with mechanical bonding (Needled - A), a geotextile with chemical bonding (Chemical - Q) and a geotextile with thermal bonding (Thermal - T). Geotextiles A and Q are made of polyester and polypropylene, respectively, while the geotextile T is made of approximately 15% polypropylene and the balance of recyclable materials from the textile industry polyester, cotton and wool. The geomembrane (G) is high-density polyethene (HDPE).

Figure 1.

Photographs of samples of studied geosynthetics, in various situations: a) to d) intact samples; e) examples of specimens subjected to punching, tensile, tearing and permeability tests; f) specimens in reservoirs with running natural sulphurous water (in continuous renovation), with evidence of the development of whitish biojelly, close to the sulphurous groundwater abstraction.

The mineral water used in this study comes from a medical spa with natural groundwater of the sulphurous type.

The geosynthetics were cut into specimens (of suitable size for the tests to be carried out), which are organized into three groups, according to the following: i) specimens submerged in reservoirs with sulphurous water without circulation (stagnant water – saw), at a constant temperature of 20°C, in the laboratory; ii) specimens submerged in reservoirs with running sulphurous water, in continuous renovation (running water – rw), at a natural temperature of 37°C, close to the spring of the Medical Spa, inside a small house dully adapted to the purpose; iii) specimens adequately stored, protected from any action that might alter their quality, i.e., a shadowy place, protected from light, humidity, and dust, at a temperature on about 20°C.

The reason for studying specimens immersed in reservoirs with standing sulphurous water is related to the need to acquire more sensibility about what can happen in terms of chemical alteration of the water after being in contact with geosynthetics. In nature, there is always continuous movement of groundwater within what is expected in a confined aquifer system associated with artesian springs. Nevertheless, on the other hand, it is more difficult to find chemical changes in the water due to the continuous renewal in running water. In any case, it is admitted that the reactive power of sulphurous water, in continuous renewal, may be much higher than that of stagnant sulphurous water, the reason why the study was carried out under these conditions.

The specimens i) and ii) were tested over 6 months and in the eighth month. The specimens iii) were only tested at the beginning, middle, and end of the works so that their results could be used as references. The physical and mechanical tests performed to the geosynthetics were mass per unit area [14], permeability tests [15], static puncture test, California bearing ratio (CBR) type [16], wide-width tensile test [17] and tearing strength test [18].

Regarding natural mineral water (sulphurous water), classical physical-chemical analyses were performed in the situations [19]: i) young water (yw), and ii) aged water (aw) after contact with the geosynthetic specimens, and in situation iii) aged without contact with any specimen, to the results as a reference.

Sulphurous waters are very common in a large group of Portuguese medical spas. As a rule, these waters in nature have pH > 7, SiO₂ > 10% concerning total mineralization, F⁻ > 5 mg/L, Sulphur in reduced, unstable forms (HS⁻ e S₂ O₃²⁻), and the presence of HCO₃₋ and Na⁺ as the dominant anion and cation, respectively. Other important elements, such as Cl⁻, SO₄²⁻, CO₃²⁻ and H₃SiO₄⁻ in the anions, and Ca²⁺, K⁺, Mg²⁺, and Li⁺ in the cations. Some gases such as CO₂ and ²²²Rn are also present, and some trace chemical elements such as Mn²⁺, Br²⁻, B₃O₃, W, Zn, Sb, and Mo.

This type of sulphurous water, if not contaminated by external agents, has excellent stability over time, as shown by several authors [20, 21]. To have a particular notion of the trace of chemical elements of the water understudy, the results of a detailed physical-chemical analysis [22] of the same groundwater abstraction that serves as the basis for this study are presented.

It should be noted that these waters aim to develop a whitish gelatinous residue (biojelly) which is typical of this type of sulphurous water (Figure 1f).

Scanning Electron Microscopy and Elemental Analysis techniques were also used in the characterization of the geosynthetic materials, before and after the contact with sulphurous water, as well as in the characterization of the biojelly. The equipment used is from the Laboratory of the Optics Centre of the University of Beira Interior. The scanning electron microscope is Hitachi, model S-2700, resolution 40 Ǻ. The detector for elemental analysis, through energy dispersive RX, is a Rontec.

3. Results and interpretation

3.1 Physical and mechanical tests on geosynthetics

Physical and mechanical tests were essential, especially to evaluate the durability of these materials when in contact with this type of sulphurous groundwater. In addition, if their characteristics evolve, some chemical elements of these materials will probably become part of the chemical composition of the waters where they are installed, namely in new trace chemical elements.

About the physical and mechanical results, the characteristics of geosynthetics over the 8 months of research were presented in previous works [8, 9, 10, 11, 12]. It was shown that the geosynthetics immersed in these environments have evolution, namely their strength, in most situations studied, decreases over time.

Table 1 summarizes the main trends over time regarding the parameters obtained in the various tests. It is mentioned that, regarding strength parameters, although it was the thermal geotextile that suffered the highest rates of decrease (−3.99%/month) of Ts, in T-rw), there are situations in those they even gained strength (+0.95%/month of α _f, in T – sw). Other increases also occurred in the geotextiles A (e.g. +4.01%/month of Ts and + 0.71%/month of Fp). In the case of Geotextiles Q and Geomembranes G, the strength parameters in total always showed decreasing rates over time, with the most difficult situation in G, with −2.33%/month of α_f, in rw situations.

Geos.	Situation	Parameter - equation	%/ month	Geos.	Situation	Parameter - equation	% / month
A	rw	μA = + 0.974 t + 155.7	0.63	T	rw	μA = − 7.345 t + 594.0	−1.24
	sw	μA = − 0.311 t + 154.3	−0.20		Rw	μA = − 5.208 t + 600.5	−0.87
	rw	Fp = + 0.010 t + 1.4	0.71		rw	Fp = − 0.001 t + 0.2	−0.50
	sw	Fp = − 0.017 t + 1.5	−1.13		sw	Fp = − 0.009 t + 0.3	−3.00
	rw	αf = − 0.004 t + 6.4	−0.06		rw	αf = − 0.080 t + 2.2	−3.64
	sw	αf = − 0.044 t + 6.4	−0.69		sw	αf = + 0.019 t + 2.0	0.95
	rw	Ts = + 7.926 t + 197.5	4.01		rw	Ts = − 3.003 t + 75.3	−3.99
	sw	Ts = − 0.0287 t + 231.9	−0.01		sw	Ts = − 1.210 t + 71.2	−1.69
Q	rw	μA = + 0.404 t + 154.5	0.26	G	rw	μA = − 1.174 t + 988.1	−0.12
	sw	μA = − 0.255 t + 157.4	−0.16		sw	μA = 3.353 t + 974.4	0.34
	rw	Fp = − 0.011 t + 2.2	−0.50		rw	Fp = − 0.027 t + 2.8	−0.95
	sw	Fp = − 0.033 t + 2.3	−1.43		sw	Fp = − 0.030 t + 2.9	−1.05
	rw	αf = − 0.010 t + 8.9	−0.11		rw	αf = − 0.504 t + 21.5	−2.33
	sw	αf = −0.141 t + 9.8	−1.44		sw	αf = − 0.285 t + 20.6	−1.38
	rw	Ts = − 0.047 t + 293.2	- 0.02	μA - mass per unit area (g/m^2), Fp - maximum plunger force (kN), αf - tensile strength (kN/m), Ts - maximum tearing strength (N), t - time (month:1, 2 .., n).
	sw	Ts = − 1.448 t + 304.3	−0.48

Table 1.

Trends obtained from the results of physical and mechanical tests overtime on geosynthetics (geos.) immersed in running water (rw) and stagnant water (sw) [12].

An interesting particularity was verified in the results of the permeability type test, verified in the geotextiles over time. In terms of velocity index (Iv), the results in rw and sw environments, over the time, are presented in Figure 2. The geotextiles in contact with the sulphurous water become more permeable during the first months (Iv increased), which is a consequence of the aggressiveness of the sulphurous water, assuming that may have the possibility that some chemical reactions occurred between the geosynthetics and the sulphurous water.

Figure 2.

Evolution of velocity index (iv) from permeability tests in geotextiles immersed in water over 8 months.

However, a particular situation occurred after some time, which was the inversion of the tendency until then verified, that is, the permeability decreased again, verified by the successive decrease of Iv; this inversion situation was followed by the appearance of a whitish residue that was developing, called “biojelly”, which was more evident in the situation of running water, at the end of the fourth month; the biojelly, in besides to filling the geotextiles by external deposition, also blocks them internally.

A view of biojelly can be seen in Figure 1f, with the more evident development, in the borders of the reservoir, where the current sulphurous water that drains directly by artesian flow from the abstraction is discharged at the bottom of the container and flow on the top. The geotextiles in the case of the sw environment also developed biojelly, despite at a much lower rate than in the case of the rw environment.

Biojelly is a typical natural product of sulphurous groundwater, practically unknown by the community of professionals who apply geosynthetics in underground works. Studies on such product are practically non-existent, deserving reference the work of Calado [23] on the deep aquifer systems of this type of water, where several times he mentions the biojelly as a particularity of this type of groundwater.

3.2 Chemical composition of waters

The results of the physical-chemical analyses of the groundwater understudy and stored over 8 months for the situations “without contact” and “with contact” with the geosynthetics are presented in Table 2. The same table also presents the results of physical-chemical analysis of the same groundwater, but in the “young situation” (young water- yw), to facilitate their interpretation. The water referred to as “young situation” is understood as water collected immediately after its resurgence from the natural mineral water aquifer and without contact with the geosynthetics. In a graphic format, Figures 3 and 4 present the results of the main physical-chemical parameters of the groundwater understudy in different situations.

Parameter		Young water, without contact		Aged water ^(2) (3)
				Without contact - aw	After contact with geosynthetics
		yw ^(1)	yw ^(2)		aw-A	aw-Q	aw-T	aw-G
pH		8.84	8.74	7.28	7.92	7.11	6.3	8.14
Conductivity -μ S/cm		292	281	318	326	330	372	320
Total Alkalinity (in HCl 0.1 N) - mL/L		16.7	15.5	15.9	16	16.4	22.9	16.1
Dry residue (at 180°C) - mg/L		226	211	249	246	255	340	238
Total Hardness (p.p.10^5 CaCO3)		0.92	0.90	1.0	1.3	1.5	3.2	1.1
Total sulphide (in I2 0.01 N) - mL/L		11.2	11.1	<1.2	<1.2	<1.2	<1.2	<1.2
Silica (SiO2) - mg/L		53.2	52.0	58.0	56.0	56.0	62.0	57.0
Total mineralization - mg /L		264	254	287	291	294	348	287
	Sodium (Na^+)	67.1	63.0	70	71	71	77	70
	Calcium (Ca^2+)	3.1	3.4	3.9	4.9	5.5	9.2	4
Cations	Potassium (K^+)	1.9	1.9	1.9	2.2	1.9	5.4	1.9
(mg/L)	Magnesium (Mg^2+)	0.12	0.17	0.15	0.18	0.21	2.1	0.16
	Lithium (Li^+)	0.30	0.27	0.3	0.29	0.31	0.3	0.3
	Ammonium (NH4^+)	0.04	0.17	n.d.	n.d.	n.d.	0.08	n.d.
	Bicarbonate (HCO3^−)	83.0	82.7	97.0	97.8	100	140	98.4
	Chloride (Cl^−)	25.2	22.1	26.8	26.8	27.4	31.3	26.6
	Sulphate (SO4^2−)	6.9	7.6	13.8	14.5	16.3	<0.3	13.8
Anions	Fluoride (F^−)	15.0	14.0	14.7	14.9	14.9	20.3	14.8
(mg/L)	Nitrate (NO3^−)	< 0.10	< 0.3	<0.3	2.6	<0.3	<0.3	0.4
	Nitrite (NO2^−)	< 0.002	< 0.005	<0.005	<0.005	<0.005	<0.005	<0.005
	Hydrogensulphide (HS^−)	1.8	1.8	<0.2	<0.2	<0.2	<0.2	<0.2
	Phosphate (H2PO4^−)	58	n.d.	n.d.	n.d.	n.d.	n.d.	n.d.
	Aluminium (Al3^+)	< 20	n.d.	n.d.	n.d.	n.d.	n.d.	n.d.
	Manganese (Mn^2+)	13	n.d.	n.d.	n.d.	n.d.	n.d.	n.d.
	Bromide (Br^2−)	264	n.d.	n.d.	n.d.	n.d.	n.d.	n.d.
	Boron (B3O3)	104	n.d.	n.d.	n.d.	n.d.	n.d.	n.d.
	Beryllium (Be)	0.8	n.d.	n.d.	n.d.	n.d.	n.d.	n.d.
	Lead (Pb)	6	n.d.	n.d.	n.d.	n.d.	n.d.	n.d.
	Silver (Ag)	< 0.8	n.d.	n.d.	n.d.	n.d.	n.d.	n.d.
	Cadmium (Cd)	0.3	n.d.	n.d.	n.d.	n.d.	n.d.	n.d.
	Vanadium (V)	< 3	n.d.	n.d.	n.d.	n.d.	n.d.	n.d.
	Yttrium (Y)	< 0.1	n.d.	n.d.	n.d.	n.d.	n.d.	n.d.
Trace	Tin (Sn)	< 3	n.d.	n.d.	n.d.	n.d.	n.d.	n.d.
chemical	Chromium (Cr)	< 2	n.d.	n.d.	n.d.	n.d.	n.d.	n.d.
elements	Iron (Fe^2+)	< 25	< 30	< 30	90	50	100	<.30
	Barium (Ba^2+)	< 2	n.d.	n.d.	n.d.	n.d.	n.d.	n.d.
(mg/L)	Iodide (I^−)	< 1	n.d.	n.d.	n.d.	n.d.	n.d.	n.d.
x 10^−3	Arsenic (As2O3)	29	n.d.	n.d.	n.d.	n.d.	n.d.	n.d.
	Tungsten (W)	28	n.d.	n.d.	n.d.	n.d.	n.d.	n.d.
	Copper (Cu)	< 1	n.d.	n.d.	n.d.	n.d.	n.d.	n.d.
	Zinc (Zn)	13	n.d.	n.d.	n.d.	n.d.	n.d.	n.d.
	Antimonium (Sb)	8	n.d.	n.d.	n.d.	n.d.	n.d.	n.d.
	Nickel (Ni)	< 3	n.d.	n.d.	n.d.	n.d.	n.d.	n.d.
	Cobalt (Co)	< 1	n.d.	n.d.	n.d.	n.d.	n.d.	n.d.
	Niobium (Nb)	< 0.5	n.d.	n.d.	n.d.	n.d.	n.d.	n.d.
	Molybdenum (Mo)	6	n.d.	n.d.	n.d.	n.d.	n.d.	n.d.

Table 2.

The physical-chemical analyses of the sulphurous water studied, before (young water - yw), and after contact (aged water - aw) with the geosynthetics.

⁽¹⁾ complete physical-chemical analysis carried out by the IGM Laboratory [22]. ⁽²⁾ summary physical-chemical analysis carried out by the IST Laboratory; ⁽³⁾ analyses performed after 8 months of aging; n.d. – parameter not determined.

Figure 3.

Results of the global physical-chemical parameters of the groundwater understudy, in different situations: i) as young water without contact with the geosynthetics (yw); ii) as aged water without contact (aw); iii) in aged water situation with contact of the geosynthetics, needled (aw-a), chemical (aw-Q), thermal aw-T and geomembrane (aw-G).

Figure 4.

Results of the main ions of the groundwater understudy, in different situations: i) as young water without contact with the geosynthetics (yw); ii) as aged water without contact (aw); iii) in aged water situation with contact of the geosynthetics, needled (aw-a), chemical (aw-Q), thermal aw-T and geomembrane (aw-G).

Regarding the global physical-chemical parameters (Figure 3), the first significant singularity is that the total sulphide parameter only exists in the yw situation to annul itself when the water is aged, even without contact with any geosynthetic. Then, it refers to the pH situation, which decreases with the aging of the water, being lower in the situation of contact with geotextile T; therefore, the basic water in the young situation changes to acidic or less basic in the aged situations. Following refers to the fact that the parameters total alkalinity and total hardness do not change much, except for aw-T, where these parameters are much higher in aged water than in young water. Finally, we emphasize the situations of conductivity, dry residue, total mineralization, and silica, in which these parameters are always higher in the aged water, including the water that has no contact with the geosynthetics.

Regarding the cations (Figure 4a), in global terms, there is an increment in the various parameters on aged water, especially water associated with geotextile T (aw-T), with significant increases concerning young water, in magnesium, calcium and potassium. Note the particularity of aged water “without contact” and “with contact” with other geosynthetics (A, Q , G), not having any notable differences. About anions (Figure 4b), there is an increment of these for most situations in aged water about the situation of young water, that is, in the following parameters: bicarbonate, chloride and fluoride; there is the exception and singularity that in aw-T, sulphate is annulled. Still, on cations (Table 2), the positive fact is emphasized that, in any of the aged waters, nitrites were never detected, and the case of nitrates were only detected in aw-A and aw-G.

Concerning trace chemical elements, the only element investigated was the Iron, and it looks that the aged water does not gain this element without contact, nor by the aged water that is in contact with the geomembrane; on the other hand, the increase of this same element in the water in contact with the various geotextiles, namely in aw-T and aw-A, is very significant.

The water that has the most modifications when aged is the one that is in contact with the thermal geotextile (aw-T). From the analysis of the water classifications in the various situations (Table 3), the aged water either “with contact” and “without contact” with the geosynthetics loses in the classification, about the young water, the term “sulphurous”. A situation that still needs to be highlighted is that water with thermal geotextile changes in its classification the term from “alkaline reaction” to “acid reaction”.

Sample	Water type – Classification
yw	Weakly mineralized water, sulphurous, fluoridated, with alkaline reaction and soft
aw	Weakly mineralized water, fluoridated, with alkaline reaction and soft
aw-A	Weakly mineralized water, fluoridated, with alkaline reaction and soft
aw-Q	Weakly mineralized water, fluoridated, with alkaline reaction and soft
aw-T	Weakly mineralized water, fluoridated, with acid reaction and soft
aw-G	Weakly mineralized water, fluoridated, with alkaline reaction and soft

Table 3.

Chemical classifications of the groundwater understudy in different situations: Without contact with geosynthetics, in (yw), and aged without contact with geosynthetics (aw), and aged in contact with the geosynthetics: Needled (aw-a), chemical (aw-Q), thermal (aw-T) and geomembrane (aw-G).

When performing the Piper diagram for all analyses (Figure 5), it is verified that the aged waters remain in the same group I, as the young water, that is, they are all Sodium bicarbonate type. However, the aged water next to the type T geosynthetic (aw-T) stands out.

Figure 5.

Piper diagram from the physical-chemical analyses of the groundwater understudy, before (yw) and after (aw) of the contact with the geosynthetics immersed during 8 months.

3.3 Scanning electron microscopy and elemental analysis

To analyse the much detail as possible the relation between the biojelly and the geosynthetics, scanning electron microscope images were taken of all geosynthetics before and after contact with sulphurous water. Examples are presented in Figure 6 for the cases of geotextile A and geomembrane. Comparing the images of the various geosynthetics, before and after contact with sulphurous water, we found that the biojelly is fixed in the internal structure of the geotextiles, and more clearly in the geotextiles, A and T. A completely different situation is verified in geomembranes, where the evidence of biojelly is only superficial.

Figure 6.

Examples of geosynthetic images (A and G) obtained in the electronic scanning microscope with a 200 x zoom, before (b) and after (a), from being in contact with the sulphurous groundwater.

Regarding the elemental analyses, it was not possible to obtain results for the geotextile Q and geomembrane because the analysis technique does not result in materials with those characteristics. However, it was then possible to perform the X-ray Energy Dispersive Spectra for geotextiles A and T and the biojelly situation sampled in dense and less dense areas to enable more consistent analysis of results. Typical spectra for the case of geotextile A before and after contact with sulphurous water, as well as the biojelly, are shown in Figure 7 as examples of the results achieved.

Figure 7.

Energy dispersive X-ray spectra on elementary analysis of geotextiles a in virgin and aged situations, in contact with sulphurous water, and still only of the biojelly in a high-density zone (X - energy in keV, Y - count x10³).

Table 4 presents the global results of the various elements found in the various materials. It must be noted that carbon is always the most preponderant element both in the geosynthetics and in the biojelly itself. It is interesting to check that oxygen occurs in a significant percentage in virgin geotextile A and the biojelly and does not appear in aged geotextiles. Emphasizing that some trace elements such as Fe, Zn, Cr, Ti, Al are present in the geotextiles, those transferred to the sulphurous water could be detrimental to its quality and public health. A singularity was that geotextile A appears with Ti with some significance in the aged situation.

Sample ^(*)		Main elements	Vestigial elements
Geotextile A	virgin	C – 72%; O – 26%	Cr, Fe, Zn
	aged	C – 95%; Ti – 2%	Na, Mg, Al, Si, P, K, Ca, V, Fe
Geotextile T	virgin	C – 96.2%	Na, Mg, Al, Si, P, S, K, Ca, Ti, Fe
	aged	C – 97%	Na, Al, Si, P, S, Cl, K, Ca, Ti, Fe
Biojelly; from the running water in contact with the geosynthetics	dense	C – 63%; O – 28%; Si – 2.6%; P – 1.5%; S – 1.7%	Na, Mg, Al, Cl, K, Ca, Fe
	not much; dense	C – 68%; O – 22%; Si – 2%; P – 1.6%; S – 3%	Na, Mg, Al, Cl, K, Ca, Fe

Table 4.

Elemental analysis results from X-ray energy dispersive spectra of virgin and aged geosynthetics and biojelly samples.

^(*)

it was not possible to apply this technique to the geotextile Q and the Geomembrane.

Finally, we mention the situation of biojelly, which is composed of carbon and oxygen, is then, with some significances, S, Si, and even P.

4. Conclusions

After the question raised: “Will or will not the use of geosynthetics in contact with sulphurous groundwater potentiate the contamination of those waters in the aquifer system?” a study was carried out into three main dominations: i) study of the evolution over time, of the physical and mechanical properties of geosynthetics in contact with sulphurous water; ii) study of the chemical composition of sulphurous water before being in contact and after being aged, with or without contact with geosynthetics; iii) identification of advanced studies and characterization of geosynthetics and biojelly, as complementary to the previous domains, in order to clarify the interpretation of the results.

The materials used in the study were three non-woven geotextiles, being geotextile with mechanical bonding (Needled-A), one geotextile with chemical bonding (Chemical-Q) and another geotextile with thermal bonding (Thermal-T) and also a geomembrane (G). Almost all geosynthetics are composed of synthetic fibres, with the particularity that only the geotextile T has in its composition recyclable fibres from the textile industry, some of the natural, cotton and wool, as presented in the methodology section. The biojelly was a natural product involuntarily involved in the process, constituting a whitish creamy paste, and which arose naturally from the water, associating itself with the geosynthetics, not having initially been foreseen, for being an unknown possibility. In terms of methodology, among several details, we emphasized the fact that studies were carried out in two main methods: the case of geosynthetics always in contact with rejuvenated sulphurous water (rw) and the case of geosynthetics immersed in the same water, the situation of stagnant water (sw).

Therefore, from all studies carried out, there is to re-enforce the appearance of biojelly, whitish cream (Figure 1f), which is evident at the end of the fourth month of investigation, with a large quantity in the rw situation. This particularity allowed to obtain more knowledge about the use of geotextiles, especially in the domain of their filling but made it challenging to analyse the results concerning the primary goal of the research, as the inclusion of biojelly in these materials interferes with the physical and mechanical evolution of some geotextiles, over time. The biojelly, very plastic paste, sticks more superficially in the geomembrane and the geotextile Q. Inside the structure of the geotextiles A and T. These situations were more clearly identified with images of these materials, virgin and aged with the sulphurous water, from the electronic scanning microscope (Figure 6). The biojelly, from X-ray Dispersive Energy spectra studies (Figure 7, Table 4), is has been found that it is composed essentially by C and O, those together have on about 91%, to those are added on about 2.4% S, 2.3% Si, 1.6% P, and the following trace elements: Na, Mg, Al, Cl, K, Ca, Fe.

From the studies with physical tests carried out over time, within the first three months, aggressive evident action of the water in the geotextiles was found, making them more porous and permeable, a situation particularly well clarified in the results of the permeability tests, from the evolution of Iv over the time (Figure 2), more evident in geotextile A, which is the most permeable, then Q and finally T. At the end of the third month, approximately, the effect of the biojelly started to be noted, and, as it increased, the permeability decreased, reflecting in the lowering of the Iv index. However, degradation of the mechanical characteristics of the geosynthetics continued to occur. It is noted that the geomembrane was not naturally subjected to the permeability test, as it is an impermeable material.

Regarding the mechanical properties of geosynthetics in contact with sulphurous water over the time, it should be noted that almost in general, they evolve towards degradation, and in particular, the various strength parameters studied (Fp - maximum plunger force, α_f - tensile strength, and Ts - maximum tearing strength), decrease over the time (Table 1). The geosynthetics that suffered more changes were the thermal type (T), and the minor changes were the needled type (A), and it should be noted that the first ones reached rates of about −3.99%/month of Ts in an rw environment. In the case of the geotextile A, there is a positive singularity. In an rw environment, there was positive evolution, which means, it was found increases in strength over time, with the most significant situation of +4.01%/month of Ts in an rw environment; this kind of situation is explained by the fact that in the rw environment they have developed immense biojelly, with its intertwining with synthetic fibres; it is also admitted that the appearance of the chemical element Ti in the aged geotextile A also favours this situation. In the case of the geomembrane (G), where the biojelly was permanently restricted to the surface, the strength parameters in all situations always showed decreasing strength rates over the time. The most difficult situation was in α_f = −2.33%/month in the rw environment. The situation close to G was shown in the geotextile Q , where the biojelly did not penetrate easily into the geotextile structure, and almost always, there showed decreases in strength.

About the sulphurous water quality over the time studied only in sw environment. Several changes exist concerning virgin sulphurous water, including aged water without contact with geosynthetics. All aged waters, due to these changes, are loose in their classification: the term “sulphurous” (Table 3), due to the cancelation of the “total sulphide” (Table 2). Besides this singularity and still, about the classification, the aged waters almost all keep the same classification, as “Weakly mineralized water, fluoridated, with alkaline reaction and soft”, except for the aged water with the geotextile T, which instead of presenting an “alkaline reaction”, changes to an “acid reaction”.

A detailed analysis in chemical terms shows that some changes of quality occur in the water in the sw situation, mainly when in contact with the geosynthetics; from this analysis, the water that has the more significant change is that which is in contact with the geotextile T, followed by the Q , then the A, and finally the geomembrane. We emphasize the case of aged water associated with geotextile A and geomembrane as not having a significant change, and its quality is even close to aged water without contact with geosynthetics.

Finally, in global terms, it is emphasized that the geotextile A type and the geomembrane G that led to minor changes in sulphurous groundwater quality, which are indicated as the most favourable for the use of works in medical spas areas. In any case, it is understood that it must have more new similar studies to those presented in this paper, in more dilated periods, with a broader variety of geosynthetics, and with the realization of a wider variety of trace elements in the aged waters.

Acknowledgments

FCT – Foundation for Science and Technology supported this work with Portuguese funds within the GeoBioTec Centre (Project UIDB/04035/2020). We would like to thank Termalistur, Termas de São Pedro do Sul, E.M., S.A., Portugal, for the financial support for this publication.

Conflict of interest

The datasets generated during and/or analysed during the current study are available from the corresponding author on request after the publication of work.