## 2 Hydrogeological Setting  
The study area is the most southerly tip of China, with Qiongzhou Strait to the south, separating the peninsula from Hainan Island. The elevation of this area ranges from a few meters along the coast to 275 m in the south of this peninsula, see Figure 1. The Suixi Fault borders the area in the north, see Figure 2a. Terranes of Cambrian age and Cretaceous age constitute the basement rocks of this peninsula. Tertiary semi-consolidated sandy clay and Quaternary unconsolidated sediments, such as alluvial and lacustrine, overlie the basement rocks. The unconsolidated sediments include (1)  
Figure 1: Background setting (left) and Synthetic Aperture Radar (SAR) coverages (right) of the Leizhou Peninsula. The yellow, red, and black rectangles represent the coverage of JERS, ENVISAT, and ALOS1 respectively.  
Zhanjiang Group of Lower Pleistocene age with an upper layer of clay and a lower layer of scattered lenses of clay and coarse sand with gravel (Qlz), (2) Beihai group of middle Pleistocene age with a surface layer of clayey sand and a lower layer of sand with gravel (Q2b), (3) Recent sediments of Quaternary age with silty sand and clayey coarse sand (Q4), (4) basalt of Himalayan Period occurs in Quaternary sediments (\(\beta\)) with lots of dead craters [14], as shown in Figure 2a. In order to obtain the relationship between land subsidence and unconsolidated sediments, the soft soil thickness derived from 22 boreholes with an ordinary Kriging interpolation were generated (see Figure 2b). The soft soil thickness for each borehole was calculated as the total thickness of silt, sand and gray-clay which overlaid the upper layer of gravels above the basement [15; 16].  
The aquifer system in this peninsula can be grouped into three subsystems, shallow aquifer, middle aquifer, and deep aquifer [4; 14]. The shallow aquifer is an unconfined aquifer and mainly located at Q2b, Qlz, and Q4. Its maximum depth is 30 m. The middle aquifer occurs approximately from 30 to 200 m. The depth of the deep aquifer ranges from 200 to 500 m. The latter two kinds are confined aquifers, extending laterally to the coast. The recharge of the unconfined aquifer is mainly derived from precipitation with an annual rainfall of 1500 mm [4; 14], and infiltration of canal and reservoir. Besides the leakage recharge of confined aquifer, the deep aquifer can also be recharged directly from unconfined aquifer [14]. This is because the wide range distribution of \(\beta\) and dead craters (3694 km\({}^{2}\)), as shown in Figure 2a, has a high water content up to a depth of 310 m, deep enough to connect to the deep aquifer [14]. The discharge of the aquifer system is of two types, one is subterranean discharge into the sea and the other is artificial pumping. Groundwater is extracted from aquifers through production wells for water supply, such as agricultural irrigation, together with industrial and residential water [2; 4]. In addition, we also collected the land use map, see Figure 2c, to better interpret the relationship between land subsidence and land use in the following discussion [3].  
Figure 2: Geology and land use maps of the Leizhou Peninsula (LZP) (modified from [3; 15; 16]). (**a**) is the geology map with four kinds of compressible deposits: Beihai group of middle Pleistocene age (Q2b); basalt of Himalayan Period (\(\beta\)) with lots of dead craters represented by black dots; Zhanjiang group of Lower Pleistocene age (Qlz); recent sediments of Quaternary age (Q4); the black dots represent the dead craters in LZP; (**b**) is the distribution of soft soil thickness derived from the interpolation of 22 boreholes represented by black dots [16]; (**c**) is the land use map derived from Zhanjiang Municipal Bureau of Land and Resources (ZMBLR) [3], the inset is an enlarged view of Zhanjiang city.