The increase in consolidated gross margin for the six months was driven by the same factors as discussed above for the three months.
The increase in depreciation and amortization expenses was primarily due to the Badlands acquisition, system expansions and other assets placed in service during the last twelve months.
General and administrative expenses increased primarily due to compensation and benefits.
The increase in interest expense reflects higher borrowing levels to fund our business expansion ($11.8 million) and higher effective interest rates ($2.6 million), offset by higher interest capitalized on major capital projects ($10.2 million).
The June 2013 6⅜% Notes redemption noted above resulted in a $7.4 million loss on debt redemption.
The decrease in net income attributable to noncontrolling interests reflects the impact of lower earnings at our non-wholly owned upstream consolidated subsidiaries, primarily at our Versado and VESCO joint ventures, which were affected by operational issues.
Targa Resources Partners – Review of Segment Performance
The following discussion of segment performance includes inter-segment revenues. The Partnership views segment operating margin as an important performance measure of the core profitability of its operations. This measure is a key component of internal financial reporting and is reviewed for consistency and trend analysis. For a discussion of operating margin, see "Targa Resources Partners - Non-GAAP Financial Measures - Operating Margin." Segment operating financial results and operating statistics include the effects of intersegment transactions. These intersegment transactions have been eliminated from the consolidated presentation. For all operating statistics presented, the numerator is the total volume or sales for the period and the denominator is the number of calendar days for the period.
The Partnership reports its operations in two divisions: (i) Gathering and Processing, consisting of two reportable segments - (a) Field Gathering and Processing and (b) Coastal Gathering and Processing; and (ii) Logistics and Marketing, consisting of two reportable segments - (a) Logistics Assets and (b) Marketing and Distribution. The financial results of the Partnership's commodity hedging activities are reported in Other.