The Hydrochemical Characteristics of a Stressed Sand-Gravel Aquifer: Kazan Plain, Ankara, Turkey

Over-exploitation and sand-gravel mining affect groundwater resources in terms of both quantity and quality. Groundwater level and well yields in and around the sand-gravel pits significantly decrease. Sand-gravel mining also changes the turbidity levels and temperature of groundwater. Reduction and destruction of valuable aquifers are significant issues. In this case, the natural state of the aquifer disappears. The Kazan Plain in central Turkey is a dramatic example of these kinds of results. The productive sand-gravel aquifer in the Kazan plain has been substantially damaged due to intensive sand-gravel mining since the 1980s. Additionally, over-exploitation has caused notable declines in groundwater levels, particularly in the 2000s. This study focuses on the hydrogeochemical situation of the Kazan Plain alluvium aquifer after intensive sand-gravel mining and over-exploitation. Groundwater samples were collected seasonally in 2015, five years after the over-exploitation and heavy sand-gravel mining. The decline reached 20 m (about half of the saturated thickness of the sand-gravel aquifer) in the region where the intensive groundwater abstraction lasted until 2010. Some quarries continued to operate until 2010, but after that mining activity continued only at a minimum level. Today, groundwater quality has been significantly degraded due to the over-exploitation of sand-gravel mining and also the cessation of recharge from fresh river water.


Introduction
Over-exploitation and pollution are the most common anthropogenic effect on groundwater resources in many parts of the words [1] [2] [3] [4]. Quarrying and sand-gravel mining are even more dangerous [2] [5]- [10] because they destroy the aquifers. The combination of all the above is the worst that can happen to an aquifer.
Over-exploitation and sand-gravel mining affect groundwater resources both in terms of quantity and quality [11]. The potential impact of sand-gravel pits includes lowering of the water level due to excavations and mine dewatering, changes in turbidity levels in groundwater due to operations, temperature change (thermal impact) in groundwater due to cropping out to the surface, and contamination [5]- [10] [12]. The most significant environmental impact of sand-gravel mining includes reduction and destruction of the aquifer volume [2] [13] [14]. The decrease in aquifer volume by excavations and the destruction and filling of the fine-grained materials of the pits change the natural porosity, hydraulic conductivity, and flow net. As the vadose zone is removed and groundwater ponds occur, evaporation losses increase at considerable levels. Additionally, the aquifer is exposed to atmospheric and anthropogenic pollution. Another effect of sand gravel mining is the change in the direction of the flow with the stress on the groundwater aquifers due to the changes in the surface of the land. These changes in the land surface can increase or decrease groundwater water supply by rainwater [15].
Over-exploitation is a serious problem for groundwater resources in Turkey as in most of the countries of the world. Groundwater levels decline in most aquifers of the country due to over-exploitation. In such aquifers, temporary or continual decline of groundwater level due to insufficient recharge in dry seasons and excessive groundwater exploitation make it difficult to obtain groundwater in practical and economical ways. Furthermore, in most regions of Turkey, Quaternary sand-gravel aquifers are under the threat of sand-gravel mining; some aquifers have already been damaged or mostly excavated [2] [13]. The Kazan Plain is one of these kinds of regions. The productive sand-gravel aquifer has been substantially damaged. Since the 1980s, the aquifer has been subjected to serious damages due to sand-gravel mining. Additionally, groundwater exploitation has intensified, and over-exploitation has caused a decline in groundwater levels, particularly in the 2000s.
This study focuses on the hydrogeochemical situation in the Kazan Plain alluvium aquifer after intensive sand-gravel mining and over-exploitation. Groundwater samples were collected seasonally in 2015, five years after the over-exploitation and heavy sand-gravel mining.

Study Area
The Kazan Plain is located 25 -30 km northwest of Ankara ( Figure 1). Kazan is  Ovacayi is the mainstream in the study area, and it drains into the basin [2]. A semi-arid climate prevails in the region. Summers are hot and arid, and winters are rainy and snowy. The annual average precipitation, potential evaporation, and temperature are 375 mm, 1180 mm, and 118 C, respectively. The elevation varies from 750 to 2000 m; the average elevation is about 1000 m in the basin.
Minimum and maximum elevation in the plain is 800 and 975 m, respectively.

Sampling and Analysis
In this study, the main chemical characteristics of the groundwater in the sandgravel aquifer that was stressed from over-exploitation and sand-gravel mining were investigated. In this context, the representative groundwater wells selected for sampling were reliable. Sampling was conducted in 15 sites during seasonal periods in 2015. The study area and water sample stations are shown in Figure 1 and Figure 2. DO, pH, and EC were measured in situ using a portable instru-   [18], and excavation pits, the maximum thickness of the fluvial sand-gravel formation reached 45 m. The formation was thicker in the upper part (the section north and east of the Ankara-Istanbul highway) of the plain [2]. The original thickness was preserved in a small area that had not been destroyed by sandgravel mining.

Hydrogeological Conceptual Model
Reliable data on groundwater levels and groundwater contour maps representing the duration of the mid-1970s exist [18]. According to these data, the

Sand-Gravel Mining
Sand-gravel mining in the plain started after the 1980s and lasted, to some extent, into the 1990s. According to official records, a total of 16 sand-gravel pits became active in 1984. Some of them stopped their activity or were banned by the state in the 2000s. Large, deep pits, groundwater ponds, marshes and swamps, and stack material were seen in the plain at the beginning of the 1990s. Intensive excavations lasted until 2008 [2]. Sand-gravel mining was banned after 2006 by the local government due to its negative effects on the environment, particularly on the groundwater system. However, some quarries continued to operate until 2010. After then, mining activity continued only at a minimum. Today, the valuable sand-gravel aquifer in the plain seems to have been destroyed by the quarries. The most valuable part of the aquifer, which extends along the Ovacayi and Kurtderesi creeks, was removed. In other words, the aquifer volume was reduced by excavations, and the natural hydraulic system was changed by filling the pits with impermeable clay. Large groundwater ponds, marshes, swamps, useless material piles, and pits are now the characteristic features of the plain (Figure 3, Figure 4).    groundwater was abstracted from the sand-gravel aquifer [2]. After 2009, groundwater use decreased considerably due to decreased abstraction from the municipal drinking wells ( Figure 5).

Over-Exploitation and Droughts
According to estimates, [18], the annual groundwater yield in the sand-gravel aquifer and the neighboring Plio-Quaternary semi aquifer was 15.5 million·m 3 .
This amount was estimated before the removal of some of the aquifer by sand-gravel mining.
The hydrograph of Well 42,294 represents a region that is not affected by over-exploitation and sand-gravel mining ( Figure 6). In other words, this well represents nearly natural or original conditions. In this well, groundwater level fluctuates in a narrow range (7 -11 m) and is followed by a horizontal course.
Well, 34,106 and 34,107 are close to each other, but the trends of the hydro-  reduced to 13 to 14 million·m 3 after the drought.

Agriculture and Industrial Activities
The Kazan plain is a productive agricultural area. Since the soil, except for the lands destroyed by the quarries, is suitable for agriculture, and various crops can be grown. Sugar beets, dried beans, melons, watermelons, wheat, barley, and chickpeas are the most produced. In addition, in recent years, greenhouse cultivation has become a significant source of income for farmers. In addition to these cultivated agricultural products, cattle and poultry, beekeeping, and eggs are also produced. Kazan is also known for its famous plain melons, but the melon mostly is grown on slopes and arid lands.

Water Quality
Groundwater samples were collected as four sets in 15 sample sites, mostly from boreholes (Table 1) Table 2).     The dominant cation and anion in the upstream region of the plain are Ca and HCO 3 , respectively. Although this situation continues towards the central regions, the Mg, Na, and SO 4 ions increase, and, finally, the ions in the downstream region are close to each other.

Grouping of the Species by Using Composition Diagrams
Composition diagrams were plotted to identify the sources of each element during the concentration process. In this context, diagrams of total dissolved solids (TDS) versus Na, Ca, Mg, HCO 3 , Cl, and SO 4 were plotted (Figure 9). A linear relation was nearly achieved on the graphs of TDS versus Na, Cl, and SO 4 . TDS versus Ca and Mg values were scattered on the related graphs. Three main groups can be created according to the ion content in the TDS. The main groups were created as low, moderate, and high. From the upstream to the downstream in the plain, the waters demonstrate low, moderate, and, finally, high content of the selected species. in a narrow range.

USA Salinity and Wilcox Diagram
Samples that were collected upstream and in the vicinity of the Kazan district are mostly grouped in the C 2 S 1 class (medium salinity-low sodium hazard) in the USA salinity diagram (Figure 11(a)). Samples located near the downstream of Kazan are grouped in C3S1 (high salinity-low sodium hazard). Samples located in the central part of the plain are grouped in C 4 S 1 (high salinity and low sodium hazard), and, finally, the samples in the vicinity of Yenikent are grouped in C 3 S 2 -C 4 S 2 (high salinity-medium sodium hazard or very high salinity-medium sodium hazard). Most of the samples fall in the "good to permissible" class in the Wilcox diagram (Figure 11(b)). Waters that are under the dominant effects of the evaporite bearing lacustrine formations are grouped in the "unusable" class.

Comparison of the Chemical Data before and after the Stresses
The chemical changes before and after the stress of the intensive groundwater extraction and sand-gravel mining in the Kazan plain were investigated. The DSI (1976) report is the unique source accessible, having original data recorded in  the1960s and 1970s (Table 3). We must admit that the analysis method of the old samples is outdated and may contain some errors. However, these data that represent the groundwater system ( Figure 12)   Batik) were fresher than today. When the former and current results were compared, it is was seen that, for the current water samples, the EC values were 2.5 -4 times, Na and Na % were 2 times, sulfate was 1.5 -4 times, and SAR was 3 times higher. According to the former analysis, the irrigation water class was C 3 S 1 . The irrigation water class has turned into C 4 S 1 -C 3 S 2 and C 4 S 2 with the over-exploitation and sand-gravel mining.

Conclusions
The Kazan Plain in central Turkey is a dramatic example of a region where intensive sand-gravel mining and overexploitation took place, adding to the geogenic pollution. The productive sand-gravel aquifer in the plain has been substantially damaged due to the intensive sand-gravel mining that began in the In the middle and downstream region of the plain, surfaced salt waters come from the evaporite-bearing lacustrine formation around the alluvial aquifer. However, the water quality used to be better than today, before the alluvial aquifer was subjected to over-exploitation and sand-gravel mining. The fresher groundwaters are present in the upstream parts of the plain that is not affected by the evaporitic lacustrine formation. There is no significant difference between the former (in the 1960s and 1970s) and the current chemical data in the plain except for the downstream region. There were significant differences in the down-

Highlights
• The Kazan Plain is under the pressure of over-exploitation and dense sandgravel mining activity. • Analyzes were conducted to determine changes in the quantity and quality of groundwater.
• Water quality and level changes between 1980 and 2015.
• Groundwater quality has been significantly degraded due to the over-exploitation of sand-gravel mining.