Dieng geothermal field is located in central Java. It has more than 15 production wells and 3 injection wells. It produces about 60 MW(e) of electricity annually. In August 2007, 5.6 Ci of 125I tracer was injected in the well HCE-29. The same injection method as tritium injection in Lahendong was applied to this field. The sampling and subsequent measurements were carried out for a period of 135 d after injection.

Analysis of 125I

Isotope 125I is a low energy gamma emitter (35 keV) with a half-life of 60 d. On the basis of its half-life, 125I is a suitable radiotracer for short to mid-term reinjection into a geothermal field. However, the low energy characteristic makes 125I difficult to detect directly in the field. Pretreatment of sample and use of a

sensitive detector, i. e. liquid scintillation counter, are required in order to analyse this tracer activity.

The procedure used to analyse 125I is as follows:

• The samples (2 L) are delivered in plastic bottles and are weighed to determine their volumes accurately. A known quantity of inactive iodide (5 mg) is added to act as carrier, as well as to ensure that the final precipitate is of sufficient mass (about 10 mg) to be reliably filtered and weighed. The samples are then filtered if inspection reveals any debris or cloudiness, and NaOH is added to make the samples slightly alkaline (~pH9).

• The iodide is then oxidized to iodate with KMnO4 and allowed to stand for about 20 min. At the same time, any sulphide present (which would form Ag2S precipitate in competition with AgI) is oxidized to sulphate. A longer standing time might be used if organic matter is present or if there is a high sulphide concentration.

• The iodate (including the carrier) is then reduced back to iodide by adding an acid mixture (HNO3 and HF) followed by Na2SO3 solution. The HF is included to inhibit formation of silica, which would clog filters and interfere with the weight of the final precipitate. Sulphate is unaffected by this step, thus effectively removing sulphide interference. After standing, the solution is filtered to remove any traces of silica which might have formed.

• An excess of AgNO3 solution is added soon after the filtration to form a precipitate of AgI. Because AgI is much less soluble than AgCl, it is precipitated preferentially despite the approximately thousand-fold excess of chloride ions. However, small quantities of AgCl (and AgBr) are formed. After standing in the dark, the precipitate is filtered through cellulose acetate paper under vacuum. The AgCl and AgBr are then removed by washing with ammonia. The precipitate, now pure AgI, is then passed quickly through a further oxidation-reduction cycle for purification purposes before being dried and weighed.

• The precipitate is then dissolved in the liquid scintillation cocktail. This is done by inserting the rolled filter paper into the cocktail vial, adding about 20 mg of acidified thiourea to complex the AgI, and then immersing the vial in an ultrasonic bath to disperse the AgI into the cocktail. The paper is translucent and should be left in the vial (20 mL). The precipitates are dried and weighed.

• The analytical yield is calculated by dividing the mass of iodide in the AgI precipitate by the quantity of iodide added plus that known from prior analysis to be in the sample, typically 0.1-0.2 mg/L.

Table 16 presents the sample counting rates obtained at different production wells. It shows that during 135 d of monitoring, the 125I tracer appears at wells HCE-7A, HCE-7B, HCE-7C, HCE-9B and HCE-28A. Figure 66 shows the tracer experimental response curve obtained at the production well HCE-28A.