Introduction

Freshwater is a significant resource, and it is becoming a scarce commodity in the coastal areas where rapid population growth, intensive agriculture, prosperous tourism and industry have increased the need for freshwater resources (Trabelsi et al. 2007). In the last 30 years, groundwater salinization has become an increasing threat to the water resources in the Yang-Dai River basin. The situation is complicated due to different inputs. These include precipitation (possibly polluted or saline due to strong evaporation), seawater, ascending deep groundwater and anthropogenic sources such as wastewater or irrigation return flow (Steinich et al. 1998; Trabelsi et al. 2007).

Understanding the spatial variations in the chemical composition of groundwater is helpful to identify the different pollution sources (Mahesha and Nagaraja 1996). In the previous studies, seawater intrusion is presented as the major cause and origin of groundwater quality degradation (Xiao et al. 2014; Zhang et al. 2013). Seawater intrusion began to occur in the early 1980s and was caused by excessive exploitation of groundwater in the Yang-Dai River basin of Qinhuangdao (Han 1990). Farmland, factories and tourist areas are located on the coastal area, where the economy is developing rapidly. Agriculture depends on intensive irrigation and fertilization to improve the soil efficiency. Most of the fields have been covered with wheat and maize. Other crops include mainly seasonal crops such as vegetables but also apple plantations and peach orchards. Excessive amounts of fertilizers infiltrate into the groundwater with the irrigation return flow. Waste water emissions from factories (paper plants, fertilizer plants and food-processing mills) can cause deterioration of the groundwater. Furthermore, domestic sewage makes a contribution to groundwater salinization. These anthropogenic contaminations may result in high nitrate concentrations in the groundwater.

The purpose of this study is to discern the origin of the salinity (seawater intrusion or anthropogenic sources) in the unconfined aquifer using hydrochemical data. The research will contribute to adequate land and water planning strategies in the Yang-Dai River basin, as well as to preventing quantitative and qualitative alteration of the environmental conditions.

Description of the study area

The Yang-Dai River basin of Qinhuangdao is located in Northeast China along the Bohai Sea (Fig. 1) and consists of a coastal alluvial plain, a piedmont plain and a hilly area in the northwestern part of the region. It covers about 320 km2. There are two rivers (Yang River and Dai River) in the investigated area. The climate ranges from warm temperate monsoon to coastal semi-humid climate (Zhang 2012). The mean annual precipitation is 640 mm according to the data from Qinhuangdao meteorological station, and rainfall concentrates between June and September, accounting for about 70 % of the annual precipitation (Zhang 2012).

Fig. 1
figure 1

Location of the study area and sampling sites

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Geology and hydrogeology

The geology of the investigated area consists of Archaeozoic gneiss, late Archean to the early Proterozoic granite and Jurassic rhyolite. The bedrock is mainly gneiss and granite. The Quaternary strata were developed by alluvial, aeolian, lacustrine and residual deposit and by artificial accumulation. It can be divided into middle Pleistocene, late Pleistocene and Holocene. Its thickness varies between 5 and 80 m and mostly concentrates between 20 and 40 m (Zuo et al. 2009).

The groundwater storage in the study area is controlled by its geological structure, stratum, lithology, topography and hydrological conditions. Abundant precipitation recharges the groundwater. The distribution of groundwater and water yield property is in tight agreement with the sedimentary environment (Fig. 2). The aquifer thickness and water yield properties show big differences in different region. The thickness of the aquifer can reach 60–70 m in the central plain and less in other areas. The groundwater can be divided into bedrock fissure water and pore water in loose rock. Bedrock fissure water can be found in the hills, and pore water in loose rock wildly distributed in the plains. The permeability deepens on lithofacies. The smaller values are attributed randomly to the less permeable lithofacies; the intermediate values are attributed to the sandstone, and the most permeable values are attributed to the sand.

Fig. 2
figure 2

Simplified geological cross section through the aquifer system (Zhang 2012)

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Human activities

Human activities in the study area are mainly agriculture, industry and tourism. The cultivation of the fields began very early. Fertilizers that contain nutrients such as nitrogen, sulfate were used excessively to increase production. In recent years, the agricultural plant diversity increased, including vegetables, fruit plantations and greenhouse cultivation. Numerous factories including paper mills, fertilizer plants, food-processing factories are located in the study area. A lot of waste water is released from factories which infiltrates into the groundwater. Additionally, every May to October is the peak tourist season, which also causes a threat to groundwater as it increases domestic sewage.

Materials and methods

Sample collection and treatment

Fifteen groundwater samples were taken from domestic wells in June 2013, and one seawater sample from Bohai Sea. Well location and depth were recorded when sampled, and basic information about the cropping system and land-use type surrounding the well were also recorded. Two 50-mL polyethylene bottles with watertight caps were used to store the filtered (0.45-mm millipore membrane filter) water for cations and anions analysis. All samples were stored at 4 °C after bottling.

Electrical conductivity (EC), pH and water temperature (T) were measured in situ via an EC/pH meter (WM22EP, TOA-DKK, Japan), which was previously calibrated. The HCO3 − concentration was determined on the sampling day by titration with methyl orange as indicator and 0.01 mol/L sulfuric acid.

The major ions of the water samples were treated and analyzed in the physical and chemical analysis laboratory of the Institute of Geographic Sciences and Natural Resources Research (IGSNRR), Chinese Academy of Sciences (CAS). Cations (Na+, K+, Ca2+, Mg2+) were measured by inductively coupled plasma optical emission spectrometry (ICP-OES) (PerkinElmer Optima 5300, DV, USA). Major anion (Cl−, NO3 − and SO4 2−) analysis was carried out by ion chromatography (IC) (Shimadzu LC-10ADvp, Japan). The detection limits for ICP-OES and IC were both 1 mg/L. Analytical precision for major ions was within 1 %. For all water samples, ion balance errors (IBE) were <10 %, and most of them were <5 %.

Methods

The seawater fraction in groundwater was estimated using chloride concentration as this ion has been considered as a conservative tracer (Custodio and Bruggeman 1987), not affected by ion exchange. It is calculated as follows (Appelo and Postma 2005):

$$f_{\text{sea}} = \frac{{C_{{{\text{cl}},{\text{sample}}}} - C_{{{\text{cl}},{\text{fresh}}}} }}{{C_{{{\text{cl}},{\text{sea}}}} - C_{{{\text{cl}},{\text{fresh}}}} }}$$

where f sea is the seawater fraction, C Cl,sample is the Cl− concentration of the sample, C Cl,sea is the Cl− concentration of the Bohai Sea, and C Cl,fresh represents the Cl− concentration of the freshwater.

The freshwater sample was chosen considering the lowest measured value of the electrical conductivity (Slama et al. 2010). The only inputs are either from the aquifer matrix salts or from a salinization source like seawater intrusion, etc. (Kouzana et al. 2009). In effect, Cl− is not usually removed from the system due to its high solubility (Appelo and Postma 2005).

Based on the seawater fraction, the theoretical concentration of each ion i resulting from the conservative mixing of seawater and freshwater was calculated using:

$$C_{{i,{\text{mix}}}} = f_{\text{sea}} \times C_{{i,{\text{sea}}}} + (1 - f_{\text{sea}} ) \times C_{{i,{\text{fresh}}}}$$

where C i,sea and C i,fresh are the concentration of the ion i of the, respectively, seawater and freshwater.

For each ion i, the difference between the concentration of the conservative mixing C i,mix and the measured one C i,sample simply represents the ionic deltas resulting from any chemical reaction occurring with mixing:

$$\Delta C_{i} = C_{{i,{\text{sample}}}} - C_{{i,{\text{mix}}}}$$

When \(\Delta C_{i}\) is positive, groundwater is getting enriched for ion i, whereas a negative value of \(\Delta C_{i}\) indicates a depletion of the ion i compared to the theoretical mixing (Andersen et al. 2005).

Results

Hydrochemical characteristics of samples

The physical and chemical parameters of the 15 groundwater samples from the domestic wells and one seawater sample are listed in Table 1. The groundwater temperature ranges between 13.4 and 22.3 °C. The highest groundwater temperature was measured in the shallowest wells in the coastal plain, which indicates that the groundwater temperature in the wells was influenced by the high atmosphere temperature. The pH values range from 6.1 to 7.3. A part of the samples are slightly acidic, indicating an insignificant influence of the anthropogenic pollution from the irrigation return water or from industrial wastewaters (Table 2).

Table 1 In situ measurements and hydrochemical data for groundwater and seawater samples
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Table 2 Matrix of correlation coefficient for groundwater variables
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The water quality of each sample was plotted on a classical Piper diagram (Fig. 3). Different groundwater types were observed in the hills, the piedmont plain and the coastal alluvial plain. Ca·Mg–Cl·SO4 and Ca·Mg–HCO3·SO4 type water were found in the hills, Ca·Mg–HCO3·SO4 and Ca·Na–HCO3·Cl in the piedmont plain and Na–Cl in the coastal alluvial plain. The groundwater showed a hydrochemical evolution from Ca·Mg–HCO3·SO4 type via Ca·Na–HCO3·Cl type to Na–Cl type. The piper diagram shows that a few of samples plot near the theoretical mixing line (TML) in the piedmont plain and the coastal alluvial plain, indicating that mixing processes between seawater and freshwater were taking place. Other samples deviated from the TML due to their higher SO4 2− and Ca2+ contents. The most likely source of the sulfate is from dissolution of small amounts of gypsum scattered through the aquifer (Kouzana et al. 2009). Calcium may stem from the dissolution of calcite (Pulido-Leboeuf 2004). Thus, simple mixing, ion exchange, minerals dissolution might occur simultaneously in the aquifer system.

Fig. 3
figure 3

Piper diagram of groundwater samples

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The characteristics of groundwater salinity

The salinity of the groundwater is determined by EC (Bouchaou et al. 2008; Gime´nez and Morell 1997), which was chosen as an index to evaluate the extent of groundwater salinity (Fig. 4). The main contributors to the groundwater salinity are Cl−, Na+, Mg2+, Ca2+, K+, SO4 2−, HCO3 − and NO3 −, all of which occurred in concentrations (4.86, 4.08, 2.69, 6.16, 0.17, 2.52, 1.75 and 3.40 meq/L, respectively). EC values in the groundwater ranged from 322 to 3990 μS/cm, with an average of 1240 μS/cm. The highest values were measured in the coastal alluvial plain and in the hills. The high concentrations in the coastal alluvial plain are the results of seawater intrusion into the aquifer system from the eastern boundary of the study area, and the other high ones in the hills stem from human activities such as agriculture, industry. Also, there are many wells which receive pollution from domestic and animal effluents.

Fig. 4
figure 4

Spatial distributions of electric conductivity (EC)

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Discussion

Seawater intrusion processes

The extent of seawater intrusion is shown in Fig. 5a, b. The trend of the chloride distribution is consistent with the seawater fraction. The most serious seawater intrusion area is located in the coastal alluvial plain with a chloride concentration of 942 mg/L and a seawater proportion of 5.39 %. The drawdown from excessive exploitation of groundwater has led to seawater intrusion, and the most serious intrusion area is close to the previous Zaoyuan water source area (Zuo 2006). In the hills, the chloride concentration was up to 291 mg/L and the seawater proportion of 1.59 %, where values are higher than in the piedmont plain due to evaporation, halite dissolution and the use of chlorine-containing fertilizers.

Fig. 5
figure 5

Spatial distributions of: a chloride concentration, b seawater percentage

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Ions behave differently under the effect of seawater intrusion (de Montety et al. 2008). Therefore, a series of ionic ratio plots can explain the origin and the processes of groundwater salinization. If simple mixing between seawater and fresh groundwater takes place, there will be a perfectly linear relationship between the electrical conductivity and the chloride concentration (Milnes et al. 2006). The bivariate diagram (Fig. 6a) reveals good correlation for most samples between the electrical conductivity and the chloride concentration. Most samples scatter around the mixing line. Part of the samples are located in the chloride-enriched domain (below the mixing line), which suggests possible chloride sources from other salinization processes such as the dissolution of halite.

Fig. 6
figure 6

The bivariate diagram: a electrical conductivity/Cl relationship, b Na/Cl relationship, c Ca/Cl relationships, d Mg/Cl relationships, e SO4/Cl relationships, f Na, Ca, Mg and SO4 versus seawater percentage of groundwater samples

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Figure 6b shows the relationship between sodium and chloride. The samples also distribute around the mixing line. Few samples below the mixing line indicate that sodium is depleted in the aquifer. However, almost all samples (Fig. 6c) concentrated in the calcium enrichment domain. According to Fig. 6d, Mg2+ concentrations change distinctly at different points. The depletions of Na+, and the corresponding enrichment in Ca2+ and Mg2+ at some points, suggest cation-exchange reactions and a strong water–rock interaction related to dilution of gypsum, dolomite and calcite. The trend of Mg2+ is consistent with the trend of SO4 2− (Fig. 6e).

In order to completely understand the processes that the theoretical content variation indicates, and to find out more about the behavior of the cations, the ionic delta was plotted for sodium, calcium, magnesium and sulfate (Fig. 6f). The great majority of the samples are depleted in Na+ and enriched in Ca2+, Mg2+ and SO4 2−. This also suggests a direct cation exchange as it usually happens in the initial stage of seawater intrusion (El Yaouti et al. 2009) and the dilution of gypsum, dolomite and calcite. However, the excess of Ca2+, Mg2+ and SO4 2− indicates the existence of other sources contributing to the enrichment of groundwater. Fertilizers can be considered as potential sources of Ca2+, Mg2+ and SO4 2− (Milnes and Renard 2004).

Pollution by human activities

Due to the joint development of industry, agriculture and tourism, groundwater is under enormous pressure in the study area. Contamination by nitrate occurred. The spatial distribution of the nitrate pollution is shown in Fig. 7. Nitrate concentrations range between 35.6 and 686.5 mg/L. The majority of the study area is subjected to pollution above the drinking water standard (50 mg/L). The most serious polluted areas are concentrated in the hills and in the south coastal of the alluvial plain. Agriculture is the dominant land-use form in the hills, and lots of fertilizers with high level of NO3 − flow into groundwater. In the south coastal alluvial plain, nitrate concentration increased due to industrial wastewater, fertilizers and domestic sewage.

Fig. 7
figure 7

Spatial distributions of nitrate (NO3)

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The relationship between nitrate contamination and seawater intrusion is shown in Fig. 8a, which reveals high nitrate concentrations at low seawater fraction. This indicates that the nitrate contamination has no relation to seawater intrusion. Instead, nitrate contamination can be attributed to human activities including abuse of fertilizer, industrial wastewater and domestic sewage.

Fig. 8
figure 8

The bivariate diagram: a seawater fraction versus NO3, b chloride versus NO3/Cl, c SO4/NO3 relationship, d Ca/NO3 relationship, e Ca/SO4 relationship, f Mg/NO3 relationship, g Mg/SO4 relationship for groundwater samples

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In the case of contamination by nitrate, the ratio NO3 −/Cl− versus Cl− (Fig. 8b) can be used as an indicator to identify seawater mixing and anthropogenic pollution (Park et al. 2005). A NO3 −/Cl− ratio close to 0 suggests a high extent of seawater intrusion (Khaska et al. 2013; Zghibi et al. 2013). Reversely, a high ratio indicates pollution by human activities. Obviously, there are two group samples in Fig. 8b. A part of samples indicate not only seawater intrusion, but also nitrate contamination, which is located in the south coastal alluvial plain.

The plot of SO4 2− versus NO3 − (Fig. 8c) exhibits a certain relationship, which indicates an origin of SO4 2−and NO3 − related to either gypsum dissolution or by the contribution of SO4 2− fertilizers (Chae et al. 2004). The same situation appeared in Fig. 8d, which shows a certain relationship between Ca2+ and NO3 −, suggesting the origin of Ca2+ is both dissolution of gypsum (dolomite, calcite) and application of Ca(NO3) −2 fertilizers (Ben Moussa et al. 2008; Stigter et al. 2006). Therefore, salinization is not only due to typical seawater ions but also due to those elements contained in fertilizers like NO3 −, SO4 2−and Ca2+ that increase EC value.

Conclusion

A series of methods were used to study the origin of groundwater salinity and hydrochemical processes including ion spatial distribution, ionic deviations, piper diagram, seawater fractions and binary plots. It was found that the water type varies in different places: Ca·Mg–Cl·SO4-type water was identified in the hilly areas, Ca·Mg–SO4·Cl identified in the piedmont plain, and Na–Cl dominated in the coastal alluvial plain.

EC values varied between 322 and 3990 μS/cm, with an average of 1240 μS/cm. Higher values were found in the coastal alluvial plain and in the hills than in piedmont plain. Groundwater salinization is caused by typical seawater intrusion and anthropogenic pollution by NO3 −. The most serious seawater intrusion area is located in the coastal alluvial plain with a chloride concentration up to 942 mg/L and a chloride proportion of 5.39 %. Ion behaviors suggest that cation-exchange reactions and strong water–rock interactions related to dilution of halite, gypsum, dolomite and calcite happened during seawater intrusion.

Nitrate concentrations were found to be above the drinking water standard (50 mg/l) in most of the study area and varied between 35.6 and 686.5 mg/L. The hills and the south coastal alluvial plain suffered from severe pollution from industrial wastewater, fertilizers and domestic sewage. A part of the samples indicated simultaneous seawater intrusion and nitrate contamination, mostly located in the south of the coastal alluvial plain. Accordingly, the groundwater salinization seems to be a complex process with two main sources, seawater intrusion and anthropogenic pollution.