Acid Mine Drainage in the Chinkuashih-Shuinantung Area

The Chinkuashih-Shuinantung area is mainly composed of outcrops from the Miocene Nankang Formation. Crustal movements after the late Pliocene have led to several high-angle faults and lifted the Shitih Formation to the surf ace. During the Pleistocene, dacite intruded into Miocene sediments and thereafter hydrothermal gold-copper ore deposits were formed. These deposits are composed mostly of metal sulfides ,including pyrite, enargite, galena, tetrahedrite, and sphalerite, together with limited native gold. AU these sulfides may produce acid mine drainages. The river bed course of the Chiufen Chi is rather close to the fault traces 1 indicating a close relationship between water flow systems and tectonic movements. Water and soil samples were collected from several selected sites, including the upper 1 middle, and lower stream areas of the Chiufen Chij along with the west, south, and east banks of the Shuinantung Bay; in addition, mine waters drained directly from the Penshan 5th adit and 7th adit were also collected near their portals. Geochemical data such as pH value, water temperature, turbidity,_ conductivity, dissolved oxygen, and selected ionic concentrations, were measured . later. All samples were collected and analyzed during each season of a year. The results indicate that water drained from the two mine adits and the lower stream area of the Chiufen Chi exhibit low pH values, high metal (esp. Cu and Fe) and sulfate contents, and great conductivities. The acid mine drainage effect is very obviou's, especially during the rainy season. The Transient Electro-Magnetic (TEM) method, using square loops 20 m on a side, was also applied in this area for prospecting purposes. Measured transient voltages were inverted in terms of 1-D models. Four-layered models consisting of low-high-low-low resistivity patterns best fit the 49 coincident Jpop data. The effective exploration depth was between 5 m to 50 m. Furthernior.e, resistivities measured along the Chiufen Chi varied with. the season, indicating high porosity and permeability in tocks underlying the study area, probably due to ore mineralization· , faults or jointsjon the other hand, resistivities measured along the Shuinantung Bay are not affected by seasonal c_hange, indicating a saturated aquifer underneath the area near the sea. * Institute of Geophysics, National Central University 297

The maximum altitude of the study area is over 700 meters above sea level, in the southeastern part of this area.Figure 1 shows evenly distributed topographic contours with rather steep slopes.The Chiufen Chi is the main river in the area.Its river course and flow pattern are rather close to the known fault traces, indicating the influences of fracture zones.The outcropped strata include the Miocene Juifang Group with the Nankarig Formation and the underlying Shitih Formation.Crustal movements after the late Pliocene led to several high-angle faults (mostly normal faults striking N-S and E-W) that lifted the Shitih Formation to the surface.During Pleistocene times, dacite intruded into the Miocene sediments, and thereafter hydrothermal gold-copper ore deposits were formed (see Figure 2).The outcropped stratigraphic units are described as follows: (1) The Shitih Formation: sparsely distributed in the Chinkuashih mine and village areas, consists of gray colored, fine-to medium-grained sandstones Vol.2, No.4 M 2.00 A-Fig. 2 Geological map and ore bodies in the Chinkuashih area (after Wang, 1973).
interbedded with gray to dark shales, thin coal seams and coaly shales.
(2) The Nankang Formation: includes Nankang Sandstone and the underly ing Tzoho Formation.This formation is the most common sedimentary stratigraphic unit in this area, distributed along routes near the sea and between Chinkuashih and Shuinantung in addition to the southeastern part of this area.The Nankang Sandstones are bluish-gray colored, cal careous, thick, and of shallow-water origin.Whereas the Tzoho Formation consists of dark gray shale on top of yellowish-gray, fine-grained graywacke and is also of shallow-water origin.
The most important ore deposits in the study area are the Penshan and the Changjen ore bodies located in the southern part of this area.These de posits are composed mostly of metal sulfides, including pyrite,enargite, galena, tetrahedrite, and sphalerite, together with native gold.The sulfides are respon sible for the production of acid mine drainages.

GEOCHE MICAL ANALYSES
Sample sites are shown in Figure 3. Water samples were collected from mine.effluents near the portals of the Penshan 5th and 7th adits; the upper, middle, and lower stream sections of the Chiufen Chi; and from a small gully over the eastern Shuinantung Bay.Soil samples were also collected along the Chiufen Chi and along the Shuinantung Bay.Eleven water samples and eight soil samples were collected during esch season of a year.Water temperature, turbidity, dissolved oxygen, conductivity, and pH value were measured with a water quality detector while sampling.Total sediment (TS) and suspended sediment (SS) were determined by filtering and weighing.Chemical oxygen de mand (COD) was detected using a IIJIMA C303 COD Meter.Sulfide and NH3 contents were determined by CNS-K9070 and CNS-K9099 Spectrum Absorp tions respectively.Sulfate content was d � tected using the BaCl2 precipitation method.Total hardness (Ca and Mg) was decided by EDTA titration.Finally, metal contents including Fe, Cu, Zn, Mn, Pb were measured by Atomic Ab sorption.Soil samples were prepared by both water-wash and nitric acid-wash before analyzing.
The results of geochemical analyses ate listed in the appendix, and are summarized as follows: (1) Unit: Heter Fig. 3 Water and soil sample locations.
Mine effluents from the Penshan 5th and 7th adits exhibit strong acidity and great conductivity; high total sediment, metal(esp Cu and Fe), and sulfate contents.Furthermore, the phenomena are not affected by seasonal changes, implying an acid mine drainage effect(see Figure 4).
The effluents from the Penshan 5th adit and the Chiufen Chi are two main sources of the pollutants in the Shuinantung Bay.Both of them can be directly or in directly related to acid mine drainages in the stud y area.
(3) Metal content as analyzed from water-washed soil samples may not include all of the metal attached.On the other hand, metal content as analyzed from nitric acid-washed samples may include primary metals to a certain

., Tll
Sample sites as shown in Figure 3.

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Vol.2, No.4 extent.Therefore, results of soil sample analysis can only be compared between sample sites, on a qualitative basis.(4) Soil samples collected near the portal of the Penshan 7th adit also show strong acidity and high metal content during each season of a year.How ever, this is not so for soils collected near the portal of the Penshan 5th adit; Obviously the acid mine drainage reaction is more complete in the lower 7th adit than in the higher 5th adit.(5) Mine efH.uents contain less dissolved oxygen compared to the surface wa ters, indicating an anerobic and reductive environment in the underground workings.( 6) The chief metal pollutants in the study area are iron and copper, with zinc, manganese, and lead next; the main nonmetal pollutants in this area are sulfide sulfur and sulfates.( 7) The chemical composition of the stream water in the Chiufen Chi more or less reflects seasonal and precipitational changes.On the other hand, those from the mine drainages were nearly stable throughout the whole year.However, the change of chemical compositions in the acid mine drainage is more obvious during the rainy season, over the lower part of the Chiufen Chi.

GEOPHYSICAL PROSPECTING
From August, 1989 to May, 1990, Transient Electro-Magnetic (TEM) soundings were conducteq in the study area.The purpose of this TEM survey was to map the groundwater fl.ow system and related basement topography; A total of 49 coincident-loop TEM soundings (Figure 5) were made with 20 me ters as a loop side length.The relatively small dimensions of the transmitter array, with an effective exploration depth of between 5 m to 50 m, allowed TEM surveys to be the only proper geophysical method to operate in this hilly area with limited open spaces.
The basic principle of the TEM method • of geophysical prospecting is • shown in Figure 6.The driving current fl.owing in a transmitter loop sets up a magnetic field and induces eddy currents to fl.ow in any good electrical conductor in the ground as it is switched off.These eddy currents set up a secondary magnetic field which can be detected by a receiver loop as a time dependent decaying voltage.The recording of the "transients" is a means of detecting conductors in the ground.The decaying transient can be described by a number of measurement channels recording the voltage at various delay times during the "quiet time" between current pulses.The character of this decay (duration, amplitude, etc.) depends on the conductivity, shape and size, depth and attitude of the conductor and its position with respect to the receiver loop.
Ooloy Time ( mo ) --1!!. .,__ >-jl).l'Vol.2, No.4 It can also be used to provide information on all of these factors (Knight and Raiche, 1982;Raiche, et al., 1985;Raiche and Spies, 1981).Furthermore, the receiver-transmitter array of -the TEM does not need to be widened in order to obtain deeper subsurface information.The depth of investigation for the TEM method is mainly a function of the recorded transient time.
SIROTEM II SE (CSIRO, Australia) was used in this TEM survey.Each measurement was recorded by stacking up to 512 readings before they were printed out.These SIROTEM data were then inverted, with the aid of the Jupp-Vozoff inversion scheme (1975), to give the thickness and resistivities of the stratigraphic layers for quantitative interpretation.Four-layered models consisting of low-high-low-low resistivity patterns best fit the field data.Some of the typical field data and their inverted results were also shown in Figure 5. Consequently, without having other geophysical or geologic information strongly suggesting the use of models having more layers, we used a four-layer model for interpretations.
The inverted TEM results at different depths were contoured and shown as a depth slice map in Figure 7.In the two extremities of this area, i.e.SW and NE parts, resistivities were low, while high resistivities (more than 200 ohm-m) " predominated in between.The lower resistivities were interpreted as an over burden with a saturated aquifer while the higher resistivities indicate basement rock.From Figure 7, it is clear that the saturated aquifer was interrupted by the basement rock in the central part of the area.Moreover, the SW-NE trend     than 20 ohm -m as overburden.These maps had been rotated 45 and -45 degrees respectively.The groundwater Howing from SW to NE is rather clear: It is roughly parallel to the surface water system.
In order to check whether resistivity varied with season in the study area, some of the TEM data {Figure 9) were collected and analyzed during each season of a year.The results indicate that resistivities from the Chiufen Chi varied with the season, indicating high porosity and permeability under the study area.On the other hand, resistivities measured along the Shuinantung Bay were not affected by seasonal changes, indicating a saturated aquifer underneath the area near the the sea.
In summary, the TEM results are listed as follows: (1) The groundwater fl.ow is from southwest to northeast roughly parallel to the surface water system.(2) Water table configurations of the area study are a gentle reflection of the topography, and also generally changed with the season.(3) Using square loops 20 m on a side, four layers were mapped to a depth greater than 50 m.This demonstrates the effectiveness of the TEM tech nique for delineating both groundwater flow and basement topography, especially in hilly areas.•

CONCLUSIONS
(1) Surface water and soil were high in iron, copper and sulfate contents, especially throughout the lower stream area of the Chiufen Chi. Fig.1

Fig. 9
Fig. 9 Sites of seasonal variation of resistivities1 with some typical data shown in the Chinkuashih-Shuinantung area.
(2) Resistivities measured along the Chiufen Chi varied with the season, in dicating high porosity and permeability under the study.area, probably due to ore mineralization and/or crustal movements.Good passages for ground water can be expected throughout the area.(3)Resistivities measured along the Shuinantung Bay were not affected by seasonal change, indicating a saturated aquifer underneath the area near the sea.( 4) Acid mine drainage from the Chinkuashih is concluded to be the main reason to cause marine and river pollution over the Shuinantung area.Vol.2,No.4Appendix: The results of geochemical analyses.